lucy24 - 7:00 am on Oct 2, 2012 (gmt 0)

"I don't think you're right for this company. We'll call Friday the {two weeks from now, or shortest time prescribed by law, whichever is later} your last day." Be sure to check applicable laws; there may be special rules about how long you can make a terminated employee wait for their final paycheck.

If you're lucky, they'll say "### that-- I quit!"

Why bother with warnings? This does not sound like an employee who has a very rapid learning curve. Warnings are for overall acceptable people who occasionally do something unacceptable.