To oppose; to hold back; to resist (as one's appetite); to restrain.
The -un imp. focuses more on blocking, stopping or restraining the whole of s.t.. The -an imp. focuses more on a part of s.t. They can be used interchangeably sometimes.
If you are always opposing each other, before long your organization will be destroyed.
Resist your appetite if you don't want to run out of provisions.
Restrain that child so he doesn't fall.