Email won’t stop trying to send

I tried to email videos and got a warning that they might not be able to send due to size. I ignored that and told my phone to try anyway. Now I can’t stop my email app from trying to send the email and it’s drainig my battery I think. When I go into outbox it just spins and I can’t click on edit to delete. What should I do?

(I have tried holding down the lock and home button am my phone will not shut off that way. I read on another website to try that but I can’t.)

Thanks!