Eric,
So you don't want the lead created in Marketo OR SFDC, or just SFDC?
I'm not sure what you mean by input being different from the contact information.
If you don't want it in SFDC, just make sure that your Sync System blocks these kinds of leads by flagging them in some way - Smart List, new field that's required to Sync...
If you don't want the form to submit based on the values the lead uses, then you need to use javascript to stop the form and request different info.