Someone is spending money on my credit card

As others have stated, and linked the support thread, I'd set up 2 factor authentication as well, so someone can't just log into your account without you approving it.

https://support.xbox.com/en-US/my-account/security/compromised-account-solution

Contact the credit card company and tell them and they should help you out, you need to fix this ASAP

For future purchases, I suggest you also enable the feature to ask for a password every time a purchase is made on your Xbox ... https://support.microsoft.com/en-us/help/4028531/microsoft-store-prevent-unauthorized-purchases

You need to contact your credit card company to resolve this quickly.

Might be sensible to also ring your bank and get your card stopped. Sooner you do it, the more they can help.