you can buy these steel belt things that attatch to the cats to stop them being stolen
http://www.google.co.uk/imgres?q=catalyst+antitheft&um=1&hl=en&safe=off&sa=N&rlz=1C2CHNU_en-GBGB431GB455&biw=1440&bih=785&tbm=isch&tbnid=xVK6iD1eDJBPFM:&imgrefurl=http://www.autoaccessoriesgarage.com/Exhaust/CatClamp-Catalytic-Converter-Lock&docid=N_Jj5VPUiqWMZM&imgurl=http://www.autoaccessoriesgarage.com/img/group/main/48/4856_2_lg.jpg&w=875&h=641&ei=h7SqT96iMoXJ0QXvx52kAQ&zoom=1&iact=rc&dur=445&sig=117380945538457419645&page=1&tbnh=131&tbnw=181&start=0&ndsp=28&ved=1t:429,r:17,s:0,i:110&tx=35&ty=53