My online sub is trying to break things off. How do you think i should punish her?

Replace her with me, Ma’am!

Spreader bar and an automatic fucking machine for four hours.

Hot candle wax

Move on. That's the point where things are just not healthy. If someone doesn't want to be with you, that's the end. These things don't work any differently in d/s settings - except that in this context it's even more important to respect people's boundaries with these things.

Plenty of others out there.

You don't. If she's really determined to break it off you can't force her to continue. Even if you agreed to something like blackmail beforehand I wouldn't use it in a situation where someone really wants out as such a relationship is toxic for both and will just cause more disobedience.

The worst punishment in the world...let her and tell her that you are dismissing her. That she may never contact you again...what worse punishment t could there be?

Tell her to throw away her collar. You will have men contact her until she comes to her senses. You are the mistress, show her how you are strong

Steal her identity.

Block her and find another one maybe?

Share her! I would enjoy messing with her some! :)

I think a good spanking is in order

That is a good question