make the server unable to accept fusing (pet yard command packet id 3, make a handler for it) unless it wouldn't be a 'fail fuse', so the two pet levels acquired with the packet meet the level required. and if you really care about quality you could even grey the fuse button out (if the conditions are met, i'm not 100% but i think there already is a packet sending the client the pet levels) if you use an unobfuscated client.