It’s “too late”? Well.. this might not apply. But if you hadn’t already overstayed it you could have requested a tourist visa extension.
Also.. if you hadn’t overstayed it I think you might have been able to go to Morroco or another non schengen country and come back for another 90 days, although I’m not sure if I’m right about that. (I read somewhere that you can do this, but not for a total of more than 180 days in one year—is this correct? Does anyone know?)
Also.. you haven’t left the country yet have you? In that case you haven’t gotten the stamp yet, right? In that case i would contact the US embassy for advice about whether there’s anything you can do at this point. Maybe you could just pay a fine and it wouldn’t scar you. But, I definitely wouldn’t try to trick the system. Play by the rules and you’ll be better off!
Don’t believe anything I say because my knowledge is only based on other posts and things that I’ve read. I would get advice from the US embassy, and then take the appropriate steps with the Spanish government.