To make Ruffians "resident", when they are destroying something, build a fence around them! If you have time, you can dig water around him! Same can be done with wild pinatas, sprinkling and seedos.
WARNING: Having Ruffians will make your residents unhappy and they will be afraid to move.
Trapping Sprinkling will get Ruffians to come and destroy whatever you used to trap her.
Trapping Seedos means you cant get seeds anymore, since he cant leave and you can only get 1 per visit.
Trapping wild pinatas will cause no more of that kind of pinata to appear unless you free or make resident the one you trapped.