Hrm, Leave and Delete could be confusing for some, as it seems that deleting a convo is also a way to leave it. You'd have to maybe call it something like 'Leave and Archive' to make clear the distinction.

Alternatively, you could have it say something like 'Go Inactive,' which could do the same thing of punting it to a separate folder, you not receiving any new messages in said convo, and having your name greyed out in the participants list for others still active in the convo. Logically, this could allow people to hit a Go Active button if they so desire to rejoin the convo. I don't see why someone should be blocked from rejoining a convo if they leave it. The only reasons I can think of for not allowing someone to rejoin a convo would be possible privacy concerns relating to what the other participants say there once the other person leaves (though they should know not to discuss private things in a group convo, basic logic, they should know to take it 1 on 1 for privacy) and the potential annoyance of people trying to re-invite you to a convo you don't want to be a part of; this can be solved very simply by an Active/Inactive system wherein only you can choose your active or inactive state in a convo. This system would allow for much less confusions and greater flexibility.

Also, since it wasn't stated explicitly, it should be made so that you can't be re-invited to a convo you delete because that just doesn't make any sense at all. What's the point of deleting your incriminating steamy convo with a guild honey if she can just reinvite you?