As everyone else said, document, document, document. This person is a huge risk to patient safety which should send up red flags to everyone and it's also a risk to your hospital to have someone of this caliber in your lab.
Do you have an HR counseling policy, I've used it to document unsatisfactory technical performance or a violation of organizational values and mission. Start with a verbal warning/discussion. "No you may not do a DAT test without having your competency signed off". After the verbal (document this) then take it to a level I (or whatever your HR calls it), refusing to follow policy and procedure, unsafe patient care, tie this to something in the counseling framework. Get input from HR with this.
At the time of the Level I, reiterate that if this behavior is observed again this will take the counseling to a Level II. At this point the tech may need to go on a "highly structured performance improvement plan" which I've seen done in conjunction with the employee. Goals are set, timeframe for touch base meetings set, feed back sessions on progress. You are giving them the benefit of the doubt and this is fair to the employee in question.
Usually if the performance improvement goals aren't met, bye bye, adios, das veydanya. You can't do this alone, you need upper management and HR involvement because this could become quite unpleasant for everyone involved. Especially a patient.