I tried this a while back as I had a redundant account and when people were mentioning me in posts they were using my other account. I found that there was no way of removing the account - but by removing all the sharing options and increasing the privacy no one could find the old account by mistake.
I don't know if there is a periodic clear down or if there is some way of requesting that accounts are removed. sli would be the person who could answer that.
Hope that helps.