For me, she is just not allowed to go waving her front legs when I'm on front of her... not because she will consciously hurt me, but because she might also try it with others and they might interpret it wrong. And although I'm pretty sure she doesn't want to hurt me, doesn't mean it can't happen in an accident.
So, I reward when I'm next to her for a stretch, but not when I'm on front of her.
Tried different things when she did it in front of me, first tried to move myself to her side and reward then, but she turned with me (is starting to become a pirouette in walk). So that didn't work. And actually I ask her to back as well! Works really nicely (she happens to like backing up so that's a plus), so I reward her for the safe stuff she's offering me, the stuff I'm not happy with like stretchingin front of me, gets ignored or I tell her softly that it's a very good exercise she's doing but not when I'm on front so if she please will back up for me.
Don't know why (must be your intend) but it really works best for me to tell her that I appreciate her showing off, but that I'd rather see it in a less dangerous situation.
For me the last part was very important, since Ruphina just starts to offer stuff out of her own, I feel I shouldn't ignore her attempt to show stuff to me. When I don't say out loud (though others might think I'm completely crazy
) it's just not the same... the whole feeling I'll get from her will be "Hey, but I tried something and that's not good?
" when I explain why I don't like it, she'll happily back up and be proud nonetheless
But as long as your horse is fine with ignoring and backing up, that would be my solution as well!