I hope this message finds you well, present circumstances excluded. I am a licensed attorney with over a dozen years of practice experience. It is a pleasure to assist you today.
I will answer your questions in the order in which they appear.
Can she do this? In short, no if the accounts are joint accounts, then you have as much of a right to the funds therein as she does. Since you are married, even if she is the person paying towards the credit cards and/or filling the accounts with funds, you have equal rights to those accounts. So, she can request it but you do not legally have to oblige this request.
What steps are there to stop this? Unfortunately, this is a much more difficult question. Understand that she cannot stop you from using the accounts. However, if it is a large issue (which is the case it appears), the only way to prevent extraordinary attempts on her part would be to file for divorce and have the court issue an order relative to access to the accounts while the divorce is pending. I trust that is not something you want to do, however, legally speaking, that is the only way to handle the situation.
Let me know if you have any other questions or comments. Please also rate my answer positively (THREE OR MORE STARS) on the ratings interface on your end so I can receive credit from the site for my response.
Thank you and best wishes!