We currently don't allow players to delete their own accounts, for various reasons. However, if the account has zero playtime, it is automatically deleted after three months, to save database space. If the account has playtime, the limit is two years.
Note that if you still have access to the previous account, and want to start a new game but keep the original name, you can always apply for a name change on the older account; then register a new account under the original name. Make sure you register the new account as soon as you change the first account's name, as changes are instant and - to my knowledge - the new name becomes available again immediately following a change. Note there is a waiting period after a change before you can change an account's name again.