Find a hidden place that only you know and do it there it should work if not you can have a go at me
Always express your feelings. If you think you should tell them to stop, do so. I know if it was me, I sure as heck would.
Hello,
It can be very difficult and distracting when you are working on building something with someone and somebody else is impeding the process.
Just to do a little background, what is the reason that this third person is wanting to hinder your kiss? It sounds like she may not be comfortable with the two of you being together for some reason, maybe she has some feelings of her own that she is not admitting to, so that might be worth investigating.
If such is the case, it is best to notify her of her actions in a more sensitive way so that her feelings are not hurt, and perhaps she will understand once you explain that you and your crush are only interested in each other.
Alternatively, she may just be doing so innocently without realising that something more serious is going on. Maybe you should consider letting her know that she is without intention getting in the middle of a situation and it is bothering you.
I hope this helps.