You can edit a post for about 15 minutes after it was written and posted. After that, technically at least, it is there for ever. Usually people appreciate it if you add a post to say the problem is solved and how you did it. Then award yourself a green star to signify the matter is resolved. THE POINTS WON'T REGISTER to your credit though as you cannot reward yourself.
The hosts may sometimes consider removing a post if it is embarrassing or out of context with the forum's ideals etc, but you must remember, some posts attract other contributions that may benefit someone. They don't make a habit of cancelling them.