No, there's no strength limit to forced movement. Pull 2 means pull 2, regardless of the size or weight of the target. This means your players could also use that forced movement on a gargantuan dragon - it's up to you to work out how that works, and really, if they're up against a gargantuan dragon, they need all the help they can get.
A lot of sources of pulling don't require strength, anyway. Lots of them are magical, and that includes Thorn Whip. Remember that the flavor text of a power is only there to help make sense of the power, and your players can have the spell work completely differently (PHB1, p54, on Flavor Text). In this case, considering the turret was pulled out of a wall, it may have made sense to describe it as thorns sprouting from the wall and breaking it up so that the bolts weren't fastened to anything sturdy anymore - then the thorns shoved the turret out. Maybe an entire tree grew from behind it to do that.
As a DM, I'd be pretty happy with what happened in your session anyway. A player who plays smart is a good thing, so long as you're prepared to let them surprise you. I would have done the same and let the turret rip out of the wall. Plus, it probably made your players feel pretty good accomplishing that.
Resisting forced movement
If you really want to partially or completely prevent forced movement sometimes, you have some options. I advise you to reserve them for special occasions where it makes sense. If your player is using forced movement regularly, let them do that. If it becomes a significant impediment to your ability to challenge them, you should talk to your player rather than introducing forced movement resistance to most encounters.
Dwarves have a racial power to move X fewer squares from forced movement they might be subject to. You may want to give this to especially heavy or sturdy things.
If your players are up against Massif, the Demigod of Really Heavy Things, you might even want to let him resist 1-2 tiles of forced movement - or half of it rounded down - just for the value of the players having some palpable feedback about his weight. He would probably have this as a trait that just passively provides this resistance.
Once-off partial resistance
In the case of a turret that's fairly solidly fixed on, you might give it an encounter power to resist 5 squares of forced movement. That way, they can resist all but some very powerful forced movement - but only once. Reason it this way:
- Weak forced movement (e.g. a pull 2) can be resisted, but it will tear the turret partway from the wall. The next pull/push/slide will definitely rip the entire thing off.
- Potent forced movement, e.g. a pull 8, will still be partially resisted - but the turret will be overcome and pulled a small distance from the wall. The sheer power of this particular forced movement was eventually able to dislodge the turret all at once, after a lot of effort.
- A source of massive forced movement - e.g. an explosion that pushes it 20 squares away - would naturally send the turret flying.
The Restrained status condition prevents all movement - including forced. Someone who's restrained also grants combat advantage and has a -2 to their attack rolls.
If something's firmly fastened down, you may want to apply this condition, but without the -2 to attack rolls. Granting combat advantage still makes sense, as the restrained thing has little capacity to dodge attacks.