Can't you just sed /whatever/d /$path/local/etc/sudoers and then move the result back over the original file?

Locking issues notwithstanding, of course. Have you read the visudo and related man pages? You could screw up big time if two of these critters run over each other at roughly the same time.

vi (or rather, its bare-bones cousin ex) uses roughly the same syntax as sed, so you could figure out how to script this in vi/ex directly, and let the editor take care of locking, temporary files, and what not.