I think this is a bigger problem (ending up playing someone else riff) when you start playing guitar. Since you don't have big repertoire, you ultimately go back to what you know. I know I did this a lot.

Personnaly, I always scrap riffs that sounds like a particular artist but not that feels like one. You can play in the style of... without copying his or her riff.

I'm really not a fan of sampling. In my mind, you play their song entirely or you don't use their riff. Simple as that, but I have to admit that there seems to have a market for this so it's probably not all that bad.