I have a friend, A, in my group that is very annoying (physically). They act the same with other friends in our group, but they are okay with that. I seem to be the only one with an issue. Problem is, when I tell A not to do that to me, they don't care or dismiss me as the party pooper or similar things. What's worse, other friends jokingly say I should "give up" and let A be annoying with me, as resisting only makes it more fun for A.
This whole thing started as a joke, and I'm sure still is, I don't think A does it in bad faith. I firstly played along and laughed myself, while pushing back. It got repeated over and over to the point that it started irritating me, so I tried letting A be annoying, while not caring at all to undermine their enjoyment. Didn't work. Now the joke has gone too far for me, but I don't seem to be taken seriously when I ask to stop.
This is the kind of stuff that if a guy were to do it on a girl, it would be immediately frowned upon (or worse), but being us the same gender seems to make it not serious at all.
Beside pushing back on the specific occasions, I also wrote to A and said they need to cut with it, but they got defensive ("come on it's just a joke!") and made no difference.
I did not get a chance to confront them face to face (A did not pull that joke the few times we have been alone, seems he needs a crowd to amuse), but I'll be honest, I would have a hard time to confront A as I am not confrontational with friends and also seeing how many things have already failed has left me with little hope.
This situation is putting a toll on my mind as I keep revisiting those memories which upset me more and more and also every time I know they will be around, I get stressed.
How do I ask my friend to stop doing inappropriate things to me?
Additional info: our group is in the mid 30s.