Sorry you can't stop it. Just like dropping it in the mailbox. If it rejects you can finish it and resend it. But if it is accepted you will have to mail in an amended return to fix it. So watch your emails. You should get 2 emails, the first one confirms the transmission and the second one will say if it was accepted or rejects.
If you need help amending post back. Oh, do not change or add anything until you find out.