Not to sound mean myself here
While I agree sometimes when other people do something it can be upsetting or even frustrating. There are however some points which are confusing. For example, you mention they should go offline but at the same time when they finally do this, it's also not what's right. It feels rather conflicting to me.
I would suggest talking to and with people if something is annoying you directly, and if you feel this person has an issue with you. I mean, everyone deals with issues in their own way, but that they'd go so far as to delete stuff on their profile on dA, and leave a hidden comment seems really odd. As well as the rather passive-aggressive 'Thanks'. Not sure if it was meant or directed to you, but once again, I would suggest speaking to them. Not arguing or anything, just calmly asking and wondering why they have done that, and that they could handle the situation perhaps differently.
If it has to do with how you acted, then listen to them, ask them what you could then do differently.
Often in such situations it's that people don't communicate properly with one another. If someone has a problem with another, they should try to handle it immediately and directly, but calm and collectively.
Not sure if any of this helps, but try to talk with each other, see what's up and how to prevent these sort of situations in the future. If this person upsets you too much, either learn how to deal with how they (re)act, or keep your distance from them/break off contact.
Personally I would say first try to address the issue and work on it before taking drastic measures.