What to do if a conversation isn't interesting to me?
You can stop a conversation at any time. A simple 'I'm going to leave you, have a good one' is enough if you want to say it; you don't have to justify your decision or stay in an exchange that doesn't suit you.
Close the discussion
Closing a private message tab removes it from your discussion space. This doesn't delete the history saved between Dialova accounts and doesn't, by itself, prevent a new message. Leaving a room is a different action: it removes you from that collective space.
In case of persistence
If the person continues to contact you despite your refusal, use the block feature from their profile. Inappropriate behavior can also be reported to moderation. Reporting and blocking have different roles: one allows a problem to be examined, the other directly limits your private interactions with the person.