How do you stop Time Machine?

System Preferences->Timer Machine->uncheck "Back Up Automatically” You may need to unlock the Time Machine panel with your Admin password.

If your question is how do you stop Time Machine in the middle of a backup, select Skip This Backup. This option only appears of you have the Show Time Machine in the Menu Bar checked.

Unplugging the drive will likely corrupt it irreparably.

In addition, is there a reason you want to stop Time Machine? Time Machine works best when simply left alone to do it's job which is to back up. When users try to "out think" Time Machine or Apple engineering that is when they tend to get in trouble.