You could put an appropriate term in their contract of employment, expressly forbidding such activities. But even in the absence of such a term, all contracts contain implied terms - terms which may not be spelt out, but which are nonetheless binding. One of them is a duty on employees to serve their employer honestly and faithfully, and work with due diligence, skill and care. If you can prove that a home worker is in breach of this term, you will certainly have grounds for disciplinary action, and may - if they are working for your rivals, or the offence is repeated - have grounds for dismissal.
