There's a guy at the dump who is slow and I'm going to tell him to stop helping me with my trash.
Do you have a reason for asking him to no longer help you?
He annoys me by taking too long.
Even if you don't want their help, you shouldn't be rude to somebody who is trying to help. It is part of their job and people don't appreciate people who are rude to people who are trying to do their job.
He annoys me and I feel justified.
The man is just doing his job and he doesn't deserve to be treated poorly. You should try to be patient, but if you can't, just kindly request his offer to help and tell him it's something that you would like to do on your own. There is no need to hurt his feelings or be disrespectful.