You can edit or delete a post for about few days. You cannot delete your own post at all, if it is the first in the thread, because it would result in deletion of all the other replies.
If you want your thread closed (or deleted) for some reason, you can Report it to the moderators, as
@SabsP explains.
However, keep in mind that you don't "own" the thread and all the replies - it has become a community discussion. Just because one person got their answer does not mean that the information should be removed from the forum where it could be useful to others. After all, that is the idea behind the forum - to help one another. Some threads are continued or revived for years. Sometimes this is very interesting. Sometimes it is not - in these cases if info is outdated or there is a newer place for discussion, the moderators may then lock the thread.
Unwatching your own thread (or changing the settings so you don't watch any of your own threads) seems to be the simplest and best solution if the ongoing notifications are bothering you.