Oh yes, I have a mother like that. I remember the first 'incident'--the night before my wedding. DH and I had gone to the church for the rehearsal along with the best man and bride's maid (VERY small wedding) and did not invite my mother to join us. When we got back to my place she had 'cleaned' my apartment--the same apartment that I slaved over for 2 days before she arrived. AND she expected to be thanked!!
Go thru the garbage. Then get yourself a chest or cupboard that locks and use it! And I really like the 'No Tresspassing' sign idea.