Ex refuses to give me my stuff back?

Azn951
So, I WAS dating this girl I met back in HS and took classes with in Community College, well recently, I broke it off with her because I'm more preoccupied with school than I am with a relationship.(And the fact that she was clingy and always wanted me to see her when she knew I had so much work didn't really help either.)

Well a few days ago a close friend who works at our local mall called and told me she was in the Victoria's Secret with her ex. I thought it was just her overreacting maybe, until I called MY ex(the one who won't give me my stuff back, confirmed this). Well, I told her that it was cool that she was talking to old friends and rekindling things with her ex, but that I wanted my stuff back(shirts, school hoodie, jacket, but most importantly: a jeweled, golden necklace that my mother gave me when I was a child, you can imagine the immense sentimental value it holds for me and the trust when I gave it to my significant other). She replied a few minutes later with, "Those aren't my problems."

Now, I'm a nice guy, but if you treat me the wrong way, don't expect me to help pull you out of a fire. So should I just go over there and kick the door and just push her and her ex(probably even current) boyfriend out of the way if they're there or keep badgering her for my stuff back? What should I do? She obviously won't listen to reason as I've texted her a few times since then asking for it back. But she hasn't responded at all.
Updates
+1 y
Can I request a Civil Stand-by even though it's just an ex-girlfriend?
Ex refuses to give me my stuff back?
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