You can negate Send Time Optimization by pausing the mailing and restarting it.
Go into Analytics, Delivery statistics and click on the mailing ID. Click on Pause Mailing.
You'll get this warning message. Click on Pause again
The mailing will be paused. Give it a few minutes to cycle through and then go back into the mailing ID.
Click on Resume Mailing. This will send the rest of them that had been paused and negate the Send Time Optimization.