On a nice spring evening, get yourself a shotgun and a spotlight. Be vewwy, vewwy quiet and sit and listen. You'll spot them swimming along the banks. Blast 'em. A night spotting scope makes this easier. A full moon helps as well. Ignore PETArds that take umbrage to this. (Might consider a permit from DNR, if you want.)
When I was young, we did this several times along a creek a buddy's dad was having beaver problems on. Managed to kill quite a few in several nights and he had no troubles for the next several years.
We used rifles, but we were young and dumb. 12 ga, 3" mag turkey loads would take care of them out to 40 yards or so...
