Or you can take a high road and maybe also assume this person may not have noticed the initials on the jar. Who really reads a jar of mustard? It's yellow so it must be mustard and I need some and since noone has said anything, I guess it's for general office use.
At a new job, I was bringing 16 oz bottles of pepsi in and used the small post-it notes to indicate that they were mine. One day I hear our CEO coming down the hallway to my office calling my name. She walked in and apologized for drinking my pepsi as she had just noticed the bottles had my initials on it. I told her to not sweat it as I got them on sale at the local CVS but she was mortified. No harm, no foul. I also had a coffee cup with my name on it and while I was in the kitchen getting something, someone walks in, picks up my cup from the drainer and proceeds to pour themselves a cup of coffee. I teased them about using my coffee cup in front of me and couldn't they have waited until I was gone, and they laughed and apologized and said they hadn't realized that it really said anything. No harm, no foul! It's mustard, it's a cup.
LW, if this is such a big issue, just say something. You should have said something the first time it happened in front of you. You'd be amazed how our "worst case scenarios rarely come true and it's usually just a matter of "I'm sorry, I just didn't notice anything was written on it." Now all you've succeeded in doing is getting yourself in a snit, and embarrassing your colleague as she'll undoubtably think back to all the times she did it and wonder why you never said anything sooner.