
Rough playing can be scary when you encounter it first time in a larger breed (infact, it probably wouldn't even seem rough in a smaller pup).
I have my first mastiff pup, (he's 7 months old now :) )!!
I tried the 'yelp' when he was biting - this only got him more exited :( I tried pushing him away - he thought I was playing rough too & he got more exited :( I found the best tactic is to stay very calm, not raise my voice, use a simple, but firm 'No', then offer one of his toy to him & praise him when he chewed that. If he was really insistent I would move him off (via a firm hold on the collar), and tell him to 'sit', he would then be ignored untill he was calm.
Unfortunately, our instincts tell us to push them away & raise our voices and maybe flap our arms in and jump out of the way, but this all adds to the fun of the game for the pup. The calmer everybody can stay, the sooner she'll realise its no fun.
It happens in stages, first they will learn to bite with less force, then they will learn not to bite. At the moment Buster will occasionally 'play bite' when he is exited, but he is at the stage where there is no force to it (just lots of slobber!!!).
The key is persistence & consistency (the same reaction EVERY time) and the messege will sink in eventually.