Your tenants are not "trespassers" even if they are unlawfully detaining the property by not paying rent or otherwise breaching the lease. The Unlawful Detainer (eviction) sets forth the procedural mechanism for effecting a lawful eviction.
Even if you believe the tenant abandoned the premises you cannot renter and take possession of the property unless you follow the correct legal procedures. If you re-enter and take possession unlawfully you might be guilty of a forcible detainer. Contact me for more information.
Absent the tenant's express permission or proper legal notice you are not permitted to enter the premises. Tenant has remedies against you for entering without permission or legal process.
Unless a court order has given you back possession of the property you cannot change the locks. A tenant can get daily damages from you for the time they were locked out.
Landlords cannot interfere with a residential tenant's utility services (gas, electricity, cable, heat, etc.) in the guise of forcing tenants to vacate. Tenants have remedies against you for interrupting their utility services.
The law forbids a landlord from behavior that appears like it is intended to influence a tenant to vacate.
Before taking any action toward your tenant contact me to determine if it is legally permissible. If you already have taken legally impermissible action I can help with damage control.