So just to be clear, you're looking to pull off a con, and part of the con requires seducing the party. As a DM, you're okay with failure, but you would like a reasonable chance of success. Your conman already has an established identity which prevents them from leaving town on extended forays. And your party tends to believe that any NPC displaying a sufficient level of Interest must play into the plot somehow.
Well, first things first, you've got to figure out your angle. This is a con after all, and should be played accordingly. As someone who plays exclusively with metagamers, I will reassure you right now- this will make your job easier, not harder, as you'll see shortly.
In order to pull off a Con, you need the following components:
The Artist: The guy actually pulling off the job. If it's a complicated job, the artist might have associates working with them, but the important thing is that the artist(s) each have a specific role that they play, and the role is designed for one purpose... to convince the Mark to behave predictably.
The Mark: The target of the job. You might think that you want the Mark to believe you, but that is the simplest, most straightforward con. Your true goal is to control their behavior- while it would be easy if they just did what you said, the con will work regardless if you have the Angle.
The Angle: A motivator which the Mark will respond to predictably. Its sometimes also called the Hook, for good reason- if this were fishing, this is where you would "set the Hook." You're thinking of using romance as a motivator, and that is a classic angle, but you'll need something else.
The Kick: A play that forces the Mark's hand. I've also heard it referred to as a Hurrah, but it works the same- you give the Mark a sudden, drastic impetus, knowing that if they are sufficiently motivated by the Angle, they'll act in only one way.
So how does that apply to you and your metagamers? One of the classic Angles is to let your Mark know they're being played... But convince them that they're involved in a different con. If you can convince the Mark that they're the ones pulling off a con, they'll happily put their heads in a noose, the entire time convinced that you're going to be so surprised when they pull it off.
Let's run an example for you. Say your metagamers run into the NPC, who promptly begins throwing themselves at one of the party. This is obviously suspect, and a metagamer worth their salt (playing a "canny" adventurer) will instantly assume something is up. This is an expected response, and what you're looking for. As the con moves forward, you'll want to keep your Mark acting as expected.
You then introduce your Angle, which in this case we're going to pretend is presented by an associate. This associate "reveals" that the NPC is attempting to scam the party... and more importantly, introduces to them the idea of turning the tables. This is the most subtle step, and if possible the party should "come up with" the idea on their own. The associate can make the idea attractive by presenting their own reasons for wanting the NPC's downfall or mentioning something that they don't want, but that the PC's do... and which the NPC allegedly possesses.
If you set this up right, your PC's will buy into it precisely because they're metagamers. The way you've laid this out, they'll assume this is a quest, and they're either running with it or wrecking it. They certainly won't think them running a Con on the conman NPC is exactly what the NPC planned. At this point, the party is hooked. They'll play along with the seduction, waiting for their chance to act, and this con can otherwise work as originally planned. When the time comes for your party to play their desired role in your con, you just have to make sure your Kick convinces them they have to move on the NPC now or lose their chance (maybe it seems like the NPC is about to get caught or flee town, so they have only a small window to pull off their own con). If your "hook" is properly set, they'll gamble that they can pull off their con, act accordingly... and your plan, which relied on them acting in just such a fashion, comes to fruition. Best case scenario is that, while the party was suitably distracted, the conman hit their real target. Other possibilities are that the party's efforts were a distraction; that the party is "caught red-handed;" or even that they succeed, but were tricked into conning an innocent NPC by the associate, who was your mastermind the whole time.
This is, like I said, an example. When I've run this con on my party, I used a slightly more straightforward method, with my almighty DM powers- I made it look like the NPC was trying to seduce them as a plot point. While they were trying to figure out whether she was actually a damsel in distress or (because I'm so sneaky) an agent of the BBEG, she was making sure to show up to every major shop in town with the PC's in tow. They later discovered her missing and a ransom note left at their inn- still wondering whether she was actually in trouble (and I was just badly acting out the role) or if it was a trap, they went to the drop-off point several days away, and found... nothing. When they got back to town, they were met by the shopkeepers, who were relieved that the party had returned, because they still owed quite a bit of money. For what? Why, all the purchases the NPC had made in their name! Wasn't she with them? She had said they would be back shortly to settle up!
It took the party some time to catch up to her... But that is beside the point. The point is, you can definitely pull off a con job on a party of metagamers, so long as you count on them metagaming. The key is that they behave predictably, which you can plan for, and ultimately take advantage of.
One last example before I stop. Same party, several levels later, encountered a bard who took an immediate shine to a party member. They just as immediately assumed something was up, went in force to confront said bard, and stumbled upon a body. They were then immediately arrested for murder- they had assumed they knew what kind of con job they were walking into, and so confidently walked right into a frame job. Like I said, predictable behavior.
