depending on hwo thick it is, a razorblade should cut it just fine. if you want to shape it and you don't plan on painting it after, get abs that is designed to be heat molded. www.selectproducts.com should have some. we use heat moldable stuff at work for systems (tweeter) and it works great. if you heat regular abs to mold it, it will turn white very easily, you have to be very careful with it. the moldable stuff is much more forgiving
Well if your talkin about abs plastic...for drainage plumbing...u can use a hacksaw to cut it... or a chopsaw..sawzall...dont matter....but to mold it i don't know...im a plumber..so if we have to make a bend we use fittings with abs glue..messy as a mofo...so becareful with it...