I believe I put the wrong account number for my direct deposit. I have already filed and it has been accepted. How do I prevent someone else getting my return?

You can call the IRS at 800-829-1040, M - F, 7 a.m. - 7 p.m. to stop the direct deposit. But don't worry that you lose your refund if the name and the account number do not match then the direct deposit will not work and the IRS will mail you a check. Please see this FAQ for additional information:

https://ttlc.intuit.com/replies/3302459