If switching to thin pads isn't enough, another option to fix this problem is the Shimano CX50 cantilever.
It comes with three different size spacers for mounting the brake pads and a chart to help you figure out which one to use based on your rim width and stud separation. Notice that the pads actually attach to the brake arms behind the cantilever stud, so using the smallest spacer you can get the pad right up against the fork if you need to. For twice the price, the CX70 offers the same functionality and a nicer appearance, plus cartridge pads.