You can message someone from their profile, using the
There you can have a chat with someone. Especially useful for discussing a commission.
Currently, anyone can message anyone.
If this becomes a problem in the future, settings for limiting who can message you will be implemented. (Definitely let us know if your in need of this feature!)
If there is a more serious problem (harassment, spam, etc), report the user or let us know directly via support or however you prefer.