Answered: Jamming Opponent's Mechanism

Is it illegal to deliberately jam an opponent’s lift/drive by scoring a star or cube into it, rendering the robot useless for the rest of the match?

E.g. Team A (Red Alliance) has a six bar lift with a claw. The lift can’t lower itself enough to pick up stars if it has a cube stuck in the mechanism, so Team B (Blue Alliance), knowing this, deliberately score a cube inside of the mechanism (under where the lift would lower). This obstructs Team A’s lift, and renders their robot useless for the rest of the match.

Is this legal?

We can’t offer blanket answers on snapshots of hypothetical situations like the one you’ve proposed. That being said, what you’ve described sounds like the normal Scoring of Scoring Objects, and does not seem to violate any rules.