OK, how about something like this...
Tell the owner about the incident you saw, express your concern about damage
to aircraft and safety in general. If he demands to know the identity of the
pilots involved, tell him and let him handle it. Otherwise, suggest a
bulletin board posting or better yet a letter sent to all club
members/renters with something like 'in the past few weeks...several safety
and courtesy issues have arisen...', spell out some rules for the club/FBO
regarding taxiing and parking, and end it with '..any pilot reported
violating these safety rules will have membership/renter status revoked
immediately'.

Basically, ship up or ship out.