I had to deal with the same thing... STBXW kept coming to the house for more clothes for our D (who lives with me during the week and sees her mother on the weekend). Like you, I got upset about it at first (especially when I bought four new pairs of pants and all of them disappeared) but the truth is they're your kids' clothes, and not yours or your W's. Others may disagree, but I say just let this one go--it's not worth having the kids clothes be a proxy for the larger issues
EastTN, So your STBXW has unlimited, unsupervised access to the house you are currently living in and presumably paying for in full? The word boundaries springs to mind.
When my XW left and took half the kids clothes, the kids "assets" were divided on that day. From then on I told her to Keep "her" clothes to herself and I'll keep mine. Future purchases paid from MY money are not meant to lessen XW's financial burden.
Looking forward to a "but the truth is they're your kids' clothes, and not yours or your W's" post when she runs of with the kids' tv set and Playstation ;-)
M:46 WXW:40 T:20 M:13 D3,D8,D10 BD:11/12/16 D:12/14/16 OM confirmed 01/20/17