Yes, no, and maybe...
There are a lot questions at the end of the post and for good reason. There are a lot of things to consider.
First off, there is no concept of "lasso" in DnD 5e, at least in the current rules.
So how the "attacker" "lassos" the spell caster has a factor. If the DM decides that it is based an Athletics or Acrobatics check, there there is no attack roll. Since Levitate requires concentration, and one of the few ways to force a spell caster to make a concentration check is by damaging them. No attack role, so no damage. Even if there was an attack roll, a rope does not do damage so that still wouldn't cause a concentration check.
Potentially, an attacker could use a Rope of Climbing to lasso the caster, but it only moves 10 feet per turn while levitate can move 20 so you could force the two parties into aerial combat by making the levitated change altitude every round.
With that out of the way, there is more than one way to pull down a levitated caster.
Levitate can only work if the target is less than 500 pounds. If multiple people can grab on to the levitated, they can exceed the wight limit and cause the spell to fail.
- Pull real hard
While not in the rules, as a DM I would say that if the levitated was lassoed, the attacker could "pull real hard" to bring them down for one round. Under Strength:
"Push, Drag, or Lift. You can push, drag, or lift a weight in pounds up to twice your carrying capacity (or 30 times your Strength score). While pushing or dragging weight in excess of your carrying capacity, your speed drops to 5 feet."
I would allow this to fall under "Drag", and so long as the attackers max drag capacity was over 500, allow a Strength check contested against the spellcaster's DC.
The Druid cantrip Thorn Whip states:
You create a long, vine-like whip covered in thorns that lashes out at your command toward a creature in range. Make a melee spell attack against the target. If the attack hits, the creature takes 1d6 piercing damage, and if the creature is Large or smaller, you pull the creature up to 10 feet closer to you.
So not only does this do damage, but specific beats general. The attacker pulls the levitated closer. NOW, this is open to interpretation. The DM could say that the levitated stays at the same altitude but moves closer to the attacker. So unless they were directly below, it wouldn't change anything. Similarly, the Warlock Invocation Grasp of Hadar says:
Once on each of your turns when you hit a creature with your eldritch blast, you can move that creature in a straight line 10 feet closer to you.
Once again, damage and pulling closer (possibly down). Lightning Lure is another example of similar wording. Too many to list them all.
- If lassoed, unless the combined weight of the two parties is over 500, then the levitated stays up in the air, and possibly floating the roper also if they go higher.
- Rule of Cool you might be able to make the levitate sink for a round if the cowboy is strong enough.
- If the two are over 500, the spell ends and the levitated crashes to the ground.
- There are spells that can damage and therefore break concentration. They also inherently can, until the levitated's turn, lower the levitated by 10 feet. During the levitated's turn, they can compensate by going higher again.