Clearly your landlord is wrong in doing what you describe. He is not entitled to enter your domicile just for any reason and certainly without reasonable notice, usually at least 24 hours. He is trespassing and should be made aware of it.
Contact an attorney local to you. A stern letter from an attorney should do the trick. Barring that, your attorney may advise you to unilaterally change your locks. Whatever you do, keep careful record of the landlord's intrusions, what he has done in your apartment, and all communications between you and the landlord concerning these invasions. Hopefully you have at least one or two written notices to him to stay out of your apartment.
Consult with an attorney who can deal with the landlord directly and advise him that what he is doing is wrong.
Most attorneys will write a letter for a small fee.
The Landlord cannot enter without giving 24 hour notice. Make sure you keep records of his intrusions and as the other attorneys said, contact attorneys who will draft you a letter for a small fee.
This answer is provided solely for informational purposes only. This answer does not constitute legal advice or create an attorney-client relationship.