I sent an employee to that course many years ago also, even though I thought it was unscrupulous for them to make people pay for training that might lead to a job with them.  But why do you think your employee would go to work for "a competing tax practice?"  They're not any competition for a pro like you.

You could fire your employee with severance pay.  Then when she has completed the course, reconcile and hire her back.  Or does Block have a history of suing students who take advantage of their training?  They probably have an arbitration clause, requiring a hearing in Kansas City.