Unset an environment variable for a single command

env -u FOO somecommand

This will remove the environment variable FOO from the somecommand process' environment.

And to unset multiple variables:

env -u FOO -u FOO2 somecommand

Technically, they're not environment variables until someone exports them. But you can at least set them to empty:

FOO= some command

If removing them from the environment is enough, you can use env:

env -u FOO somecommand