We need to know the name of the film or to see a clip. It means if he can avoid the lift going down he will, is the normal daily meaning. However it very probably has a sexual overtone, going down could refer to XXX or to the man getting an eXxxx or even to the man "not not" getting an eXxxx

It sounds like he either doesn't want to share the lift with that particular person or he's not going down so instead of saying "No" he decides to be annoying and says "Not if I can help it" before deliberately closing the door for her. To say "Not if I can help it" means that you are aware that a possibility may eventuate but there is a chance that you may be able to change or prevent what happens by intervening and that is a way of announcing that you are about to take action.

Thank you, Cerere and Nanren888 for your helpful comments!

"Not if I can help it" = I prefer not to. . Darling, will you hold my obviously girly-looking handbag while I make a call. Not if I can help it. . Will you run downstairs and get us all cofees? Not if I can help it. . A wish to impose on someone, usually low effort. A preference to not comply, not have to do something, usually to avoid effort, or entanglement, or similar.

Was it "House"?