I'm assuming what you mean here is that your tenants failed to EXERCISE the purchase option in the lease agreement.
Unless something in your lease covers this (such as a bi-lateral early termination clause if they fail to exercise the purchase option by a certain date), it's not enforceable because you have already agreed to rent the property for the term of the lease. You can't change the terms without the agreement of the other party. Absent such a clause, I think you have two options:
1. negotiate something with the tenants if you really want them out early (and this means be willing to pay them to move out - the negotiation is over how much they will take to move out early). This will be your most likely route to success.
2. catch them in a default and use that to get them out through eviction. This depends on what your lease says. For example, my leases say rent is due on the first of the month and late or partial rent is not accepted. However, if you have been accepting late rent up to now, this would be difficult to enforce without first notifying them that won't be accepting late rent anymore.
Read your lease and see if they are holding up their end of the agreement. Otherwise, they will be your tenants until the end of the lease term (after all, that is what you agreed to do when you signed the lease).
The problem with this course of action is that it will probably take several months to accomplish.
Unfortunately, and you probably know this by now, this is one of those things best contemplated BEFORE the lease was signed. --68.158.xxx.xxx