I am looking after a pony at the moment and its nips. Should I try 2 stop it? I've already tried saying no firmly and because he's trained 2 the 'stop' hand signal doing that, but it doesn't work. I try 2 push him away, but he just comes back. I don't know what 2 do. Please help me!

Problem is, its not your pony and most likely hand feed treats are involved, so you stopping it only lasts as long as the owners are away and aren't reinforcing the bad behaviors. I personally HATE and never feed horses any thing by hand ever. But if training not to nip becomes an option heres the trick.
Baked potatoe- If you know how/when to create the nipping behavior, arm yourself with a nice hot baked potatoe. As he comes around to nip make sure he gets a nice bite of that. I've only had 1 horse a confirmed Biter that needed it done twice.
Now to be fair to the pony, if he is getting treats or nipping during girthing because of poor saddle fit. It would be very unfair to punish for a manmade problem.

We have a little shetland cart pony that nips a little sometimes, but he quit after awhile.

I remember Abraxis when he was a yearling was a terrible biter. The potato trick (twice) worked, but then he nipped from time to time. I broke him of that by grabbing his nose and hand twitching him. Not really hard, but just consistantly. He hated it and finally his thick skull was penetrated and he made the connection. As a colt he was the DUMBEST Arab I had ever seen! He was so dumb, I refused free breeding to him (before he was proven). Could have kicked myself in the butt for that bright decision. He grew up to become the 1975 Intl. Arabian Stock Horse Champion... Go Figure!!! FYI, most of my Stallion experience was working with him when he was young.

I've had several biters and nippers. Some very NOT-PLEASANT flesh removers!

One thing I've found VERY effective is when they reach out to start nibbling/nipping/open their mouth to bite- to kick them in the shin (cannon bone). Some folks think this is ridiculous but if you try it you'll find that it does work!
For one thing, they can't see you kicking them underneath their head SO they won't get headshy from someone smacking them upside the head lol-

For another thing, if they can see when you're going to give em a whack on the nose/head/neck, a horse will usually become headshy and/or just get quicker at biting and then jerk away.

Kicking their shins, after the 5-6 time, my biting horses would reach out as if to play with my coat, or give me a good ole chomp, and would stop and look down at their feet. It's hilarious! And it works!

Something that John Lyons teaches is you can rub their noses...and keep rubbing...and keep rubbing...and eventually...the horse gets very tired of putting their mouth/nose on you because every time they do, they get their nose rubbed and pulled and messed with...