I find a Stanley knife and scraper works fine. Cut the foam off in cubes, then lever and scrape off the blocks. Anything left behind, can be removed with a insulation foam remover or a blowtorch, scraper etc. Remember to use protective gear, in case of mishaps?.
Some people might light a fire, then throw the item on it (don't forget to remove any bungs, or you might have a ticking timebomb), the same applies to electric cables and sleeving removal, but this can cause a right old stink and black clouds in the neighbourhood?.