I don't know if this is just me not having experience playing online, but between 5 midair jumps, brawl's airdodge, and both rising pound and neutral pound being an easy way to not lose height in the air, I feel like it might be nearly impossible to challenge a puff that decides to stall at the ledge without putting yourself in an aweful position, and doesn't seem to be any stalling rule against something like this

There is a stalling rule against putting yourself in a position where the opponent can't reach you or attack you, such as sandbag stalling and any glitch that involves the top blastzone. Puff cannot stall at the ledge in this manner, so the rule that applies to this situation is "adapt"