smallest cog of cassette rubbing on seatstay
Add a spacer and re-dish. Or move some spacers from the non drive side over the the drive side (if they exist) if you don't want to open up the frame (that will require opening up the hub likely). 1mm is not a big deal. You will also have to fiddle with the limits on your derailleur and adjust shifting accordingly.

Or get a different cogset with something smaller like 11. There are lots of options for cogsets... unless you are being really perticular about the groupo. I don't know Campy stuff that well but on Shimano the smallest cog is always seperate. You could simply swap that... the jumb might reduce shifting performance but you'll get more function than you currently have.

Last option (maybe should be first) is simply to move the wheel up or down if it has dropouts that accomodate that. You'd have to adjust brake pads if you did that but that would be the simplest.