In the Mission Editor:
To ungroup a unit from his group, click on Groups (F2?) and drag and drop a blue line from him into nowhere space.
To delete a unit, put the mouse pointer on top of it and press the Delete key on your keyboard.
To delete a whole bunch of units, drag and drop around them to select them, then press the Ctrl and x keys together.
THobsons answers are correct if you want these things to happen in the middle of a mission.
Head over to the Editors Depot (link at the top of this page) and macguba's guide to mission editing for beginners. And some of the other tutorials in the Getting Started section.
Oh, and
Welcome to the forum!
*sigh* too slow again.