Laws and regulations typically only tell you what you have to do - not what you do not. Therefore, you are going to have a hard time finding "proof". The only place this may be addressed, if required, would be in the Ohio statutes. There is no requirement at the Federal level for opting-out of solicitations to existing customers.
I guess I would question why you would not have a process to support an opt-out?? If I was your customer and I called and said - don't send me this cr*p anymore - and you continued - I would leave.
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