If you change the locks, you may likely find yourself on the receiving end of a 'wrongful eviction' suit. Please to consult with and retain a qualified attorney to evict the tenants.
Based on the facts in your question as presented, it seems like the only option would be for the landlord to commence an eviction proceeding, especially if these are residential tenants. Aside from opening oneself to liability for a wrongful eviction, the landlord could also be held liable if there is any damage to the tenants' property (during the time they've been locked out).
You should retain an attorney and commence a housing court proceeding.