Fussing around with a knife to remove them is a poor way to go. As explained by [Adrian Kingsley-Hughes] on ZDNet, a deburring tool is the cheap and easy solution to the problem.

Getting a clean, square cut is essential for getting a good seal between the pipe and fittings. Hiller recommends using a PVC pipe cutter, a tool you can get for $10 to $20, for pipes that are 1.5 ...

For plastic use, just about any old deburring tool will do. They last a long time with minimal maintenance.