I'd always go for a bet to induce in this situation too. I've lost count of the number of silly bluff raises even quite passive players pull on me when I look fairly weak but they know they can't win by calling.
Like ChrisV says most villain's won't think a normal sized river bet gets any folds here but a small lead from you starts them thinking that now a monkey bluff raise can be big enough that it just might work! Maybe they make it really big just to be sure