Can I sue the credit card processor thru class action because they stole my money?

A criminal act can escalate to a class action if the defendant caused the same harm, (theft) to a large number (26-40 minimum) then a certified class action lawyer can sue with only the evidence of a few representitives of the class.
You should start with local law enforcement, then the Stae agencies The Attorney General, The Department of Financial Institutions.
The civil action aginst the processor will give you a potential return but a class action requires special conditions, whereas criminal act will stop future acts and maybe get restitution. Good Luck

It sounds as if you have some sort of business relationship with the credit card processor. If you do, the police and prosecutor likely would say there is no criminal violation when the parties to a civil contract disagree on what the terms of the contract are.

You should review your contract with your credit card processor. Those contracts can be hard to understand. However, if the credit card processor is doing "the same to other merchants", likely the credit card processor is claiming that its contracts with merchants allow it to collect those fees.

If have noticed that there are some companies that approach smaller businesses and offer those businesses credit card processing services. The owners of the smaller businesses often do not read the contracts and just sign the contracts based on the good promises of the salespersons. The contracts are generally bias in favor of the credit card processors.

You should review your specific facts with your attorney to see what legal options you have.