You're sure it wasn't an amorous sea lion? (G)
Sometimes a small riding sail on the backstay will help. But when the air is still and the current reverses, or wind blows you back against the ball...it is just going to bump.
Throwing out a smaller stern anchor
and pulling back from the ball might do it.
For dinks, there was a post online some year ago about using two pieces of heavy wall PVC tubing over a "V" bridle off the stern. The idea being that the dink can only get so close, and then the "V" (pointy end at the dink) keeps it back off your transom. Two or three bucks for a stick of PVC pipe to slide over existing lines
, if you want to try it. Recommended at that time for towing, so the dink wouldn't surf up and keep hitting the transom.
But you're sure it wasn't an amorous sea lion? You know, especially if you have a gray inflatable....(G)