Do the Riddler interrogations ever end?

They will stop spawning after you get all the remaining challenges on your map. Basically, Batman stops beating them for information interrogating them when he has all the remaining locations.

The Riddler trophy information isn't tied to specific thugs; if you accidentally knock one out, you haven't 'missed' him. Which thugs are the Riddler informants are based on spawn patterns, and when interrogated they'll give you the locations of five Riddler challenges you haven't yet recorded, as close as possible to the area you're in. As you mop them up you'll start to receive information about riddles and Joker balloons, and then about locations outside the area you're in. (Riddler informants, as far as I can tell, will never tell you the locations of cameras, but there's another way to get that information late in the game.)