I've been having people jump on my character's dead face and squat over me for years, and had them shoot off those visual effect items too, before they banned them. If squats are banned, they'll just go back to the kneel or sit emote.
It's just a part of PvP at this stage, just use it to motivate yourself to do better, or do what I do, hunt them down and do it back to them

They do it to get you mad and play worse, PvP is one of those things where you have to learn to laugh it off, or your team suffers.
It used to be that if you used an emote on someone, the text would show their name so you could report them, not sure if it still does that.