I think all the comments here have merit but would probably go with a loud 'Oi!' along with a point at the coat followed by a thumb gesturing elsewhere. I think a clear 'For fuck's sake' may also add a touch of gravitas to what you are trying to acheive

the movies I might go to were never that crowded

First: Eat popcorn with mouth open
Second: Crinkle the candy wrapper paper
Third: Keep sucking thru the straw in your drink when it's empty
Fourth: Fake falling asleep on one of them
Fifth: When faking sleep Snore
Sixth: Play with your SmartPhone but turn off sound
Seventh: If you can make yourself belch continuously, do it
Eighth: If all else fails get the usher, walk him to your seat, point out the people and tell the usher they
are making too much noise. When they start arguing with usher that you were the noisy one just look at
him/her and say,"see what I mean".
Ninth: Ask for your money back

P.S. Farting is a given.

They don't. We have reclining theater chairs that are huge. On a plane, I sit in an aisle seat.

This should scare them away.

I don't get that problem

After asking two oversized women to exchange seats with me and being shot down I asked the usher to find me another seat as it was impossible to sit between these people. They had their coats laying on me and I literally had about 10 inches of space to look through.