in most s&ps, the tenancy agreement will be assigned to the new purchaser. therefore, if the tenant has committed a breach of the agreement, you may remove him. however, practically, this may not be as simple as it sounds as you are right, some tenants refuse to budge.

maybe you could 1st send a notice of your intentions to terminate the agreement, and give the tenant a reasonable time to move out, failing which you may have no choice but to see a lawyer to get a court order to remove the tenant.