Contact the police and see what your rights are in terms of a restraining order

Think thats a bit excessive for the situation

If you own the house there’s no problem. Make sure she doesn’t have a spare set of keys

Peeping mom much

You don't just marry a person, you marry their family too. You become part of the family. So unless you are planning to move very far away once you are married, you will be spending all kinds of time with a crazy mom who clearly doesn't think you are good enough for her son and also believes she should have access to your home. Run...run far, run fast, and don't look back.

Problem is some of the best relationships over come such obticals together that been said I believe the generation before the 90s baby's are the most damaged....

Talk to your Fiance, discuss this, and how absurd it is.
Set some ground-rules.

Make it clear that his(or her?) mother will not drive you from your own house, and if they keep this shit up, they'll be banned from visiting.

Give her a gift that includes a restraining order!

How can your future MIL kick you out of your own house? And your fiance needs to set some boundaries with his mother now or she'll make your marriage hell.

Front door, or bedroom door?
Front door: restraining order, big talk with fiance
Bedroom door: big talk with fiance about her moving out

Time to talk to your SO about ground rules. obviously they need to grow up and talk to their mother and tell them what is and isn't okay in YOUR home. And if they aren't grown enough to do that, it might be time to find someone new

How on earth do you get to the point where you have to lock your MIL out? I expect she's crossed the line too often. I hope your fiancé tells her where to get off at least!