Your only way to do that would be to time out in all your games without moving.
Tournament withdrawal

When you hit 1306 it should have dropped you from the tournament. If you have an opponent and can play, then you play and win the games. Otherwise, make no moves and time out of them.....

When you hit 1306 it should have dropped you from the tournament. If you have an opponent and can play, then you play and win the games.........
In member-created tournaments the tournament director has to remove ineligible players manually before starting the tournament.

It looks like that at the begin of the tournament you met the criteria. Hence, you were eligible. While you have the option of timing out all your games this will impact your timeout ratio and may have negative consequences later. So if you really want to withdraw because you think it may be a cakewalk contact the tournament admin.

It looks like that at the begin of the tournament you met the criteria. Hence, you were eligible. While you have the option of timing out all your games this will impact your timeout ratio and may have negative consequences later. So if you really want to withdraw because you think it may be a cakewalk contact the tournament admin.
As the original poster stated he registered below 1200. Subsequently his rating increased to 1306. Subsequent to that the tournament started.
http://www.chess.com/tournament/12-less-than-1200
The tournament director did not remove him prior to manually starting the tournament or allowing it to auto-start. A tournament with a maximum rating should always be on manual start. Players should direct any unhappiness toward the tournament director.
The tournament director and can do nothing until Spitfire1958 finishes all his games. At that time he can be removed.
Spitfire1958 could also withdraw after finishing all games.

At 1153 I joined a tournament, 12 under 1200, which has just begun. As I'm now 1306 and no longer qualify, how do I withdraw without forfeiting ratings points?
Well you joined when you were Under 1200 right? so whats the harm..

At 1153 I joined a tournament, 12 under 1200, which has just begun. As I'm now 1306 and no longer qualify, how do I withdraw without forfeiting ratings points?
Well you joined when you were Under 1200 right? so whats the harm..
One of the players left a note complaining that it was meant for under 1200's, normally I would just have played. Timeout's fair, I reckon.

You cannot withdraw without losing points. You joined the tournament when you were under 1200. You did nothing wrong and will not be doing anything wrong by playing in that tourney. Stay and play!!

You cannot withdraw without losing points. You joined the tournament when you were under 1200. You did nothing wrong and will not be doing anything wrong by playing in that tourney. Stay and play!!
well said. I could not agree more.

At 1153 I joined a tournament, 12 under 1200, which has just begun. As I'm now 1306 and no longer qualify, how do I withdraw without forfeiting ratings points?
so what? you joined you where under the score, therefor your good to go! just because you win and your score went up after that, does that mean if anyone while playing the tournament goes above 1200 (im playing 25 games at one time (fair enough im loosing 23 of them lol, joke)) because potentially my score might increase to over 1200 before the tournament finishes? i think he should be allowed to play,
At 1153 I joined a tournament, 12 under 1200, which has just begun. As I'm now 1306 and no longer qualify, how do I withdraw without forfeiting ratings points?