You might try snagging them with spoonbill gear, it does work, ive caught them before.
Other then that, are they creating any damage, so that just letting them go would be a problem? even tho they are not as voracious eaters when they get older, they still gotta eat, and keeping a 15 20 lbr alive takes good bit of vegetation.
Ive always had a hard time figuring out why a bigger one would quit eating altogether.
