After a gig you can tell the agency how it went. It takes a moment and it's the single most useful thing you can do to improve what you're offered next.
Feedback shapes which acts the agency suggests for you. If an act went down well, say so: you'll see them again. If they weren't right for your room, that's just as useful, and it isn't a public review.
Keep it honest and fair, and about the performance rather than the person.
If you'd rather not have a particular act back, tell the agency directly; they can record that preference so it doesn't come up again.