Although it seems kind of unnecessary, you can just stop using the account, and that way if you ever did want to return and post again it is still there for you to use.
I've joined several various forums over the years, and this is the only one I've seen where people publicly ask to have their account removed. What's up with that? Is there some special benefit you can get by having your account deleted that you can't get by just not coming here anymore, or is just spurious drama?
You see threads like this all over CB. My question is why? It really is pointless to announce publicly on a website that "you no longer have a BB" or "I'm done with BB's I going to androidcentral" etc etc. I just tell them to go. No need for formalities or sad goodbyes just go. Its not like they are PK or someone notable on CB. Come on, thats like me announcing that I was leaving CB. Would anyone care? No. Would I expect them to care? No.
I guess they do it to feel important to themselves.