On mine, which was a new, but freight damaged canoe, many of the ribs were "glued" to the planking by varnish that had seeped in between them, making removal difficult without damaging the planking after grinding the tacks as suggested above (which I highly recommend). I found that an oscillating multi-tool with a straight saw blade was great at allowing me to saw the ribs into small sections that could then be removed by slipping a putty knife underneath them. This probably isn't as much of an issue with an older canoe. The same tool was also great for trimming rib tops and other delicate sawing tasks.