I'm not sure how the rent-control issue would effect things, as I've never managed a rent-control property but...
Even without a written lease, you can still initiate an eviction proceeding. And with what is going on... I think you need to clean house. I have unfortunately had to deal with squatters before, and the process was as follows:
1. Serve notice to vacate/terminate month-to-month tenancy. Required time frame here (in Ohio) is 30-day notice. Keep in mind if your tenants get with the program during this notice period, you do not have to move forward with the eviction. Also, be prepared for all kinds of problems during the 30 days and up until you get possession of the unit back.
2. Service notice to vacate/eviction. Required time frame here is 3-days.
3. File eviction and move forward.
I would consult your attorney, as eviction process varies widely. With regards to tenants changing the locks, you do have the right to change them back just be sure to be prepared to provide them with a key and entry upon demand (even if its at 2am). Also, be sure to document EVERY SINGLE lease violation. Take pictures. Write statements for your files. Send your tenants written notice with copies of the notice in the lease file. If there are issues with building code or law enforcement, keep copies of the code violations and police reports. You will need them come time for court.