I don't know if this will help, but our LO has finally learnt what "gentle" means.

When he goes to hit, bite, or play too rough I calmly (took some time to get to calm) say: Robert, gentle. He stops what he's doing and will either pat or cuddle (depending if it's a book, toy or person) and say "gentle". Then it's praise-praise-praise.

With DS1 (when he was older, 3-5) I would count him down (using 1-2-3). One was a warning, 2 was a second warning - he knew that if I got to 3 he was in "serious trouble", ie, time-out; toys taken away, mummy being cross etc. The important thing was to follow through and be consistent. After he started to comprehend that his actions had consequences it was only the rare occasion that we got past 2 and three quarters.

Good luck!