Issue the student a verbal warning. List specific problems, adequate solutions, and give the student the opportunity to improve their work practices.
If the student does not improve, issue them a written warning(e-mail is perfectly acceptable). List specific problems, adequate solutions, and the ways in which the student has failed to improve since the previous warning. Issue the student a specific timeframe in which to improve. Save a copy of this correspondence for your records.
If the student still does not improve, you are free to terminate the student. They must be notified in clear terms that the position is no longer available to them and reminded of the reasons why. Student Financial Assistance must be notified of this immediately.