I'm assuming those old t-shirts are in boxes of 100 in varying sizes in Roger's garage, so the wording is, like the poor, always with us.
Unless you can apply some iron-on vinyl cover tape. How about obscuring the last word to have "Buy this shirt, you"?
Or the first word could be covered to leave "this shirt, you idiot!"
If only people would consult me before they rushed all giddy into these business ventures, the world would be a much happier place.