I'm very curious to the answer you get.. I've been in the same predicament. If I can at all help it when people want to ride/ski with me I just forget to call them before I go.
If it's someone I am at the mountain with I have often been tempted to say hey lets ski these woods for a while, I want to get a good 'workout' in so I'll meet you @ the bottom of the lift in a while.
Maybe a hey I'm gonna lap this- we'll ski these woods for a while, or this lift for a while if you wanna go someplace else throw me a text and we'll meet back up!
hah, reminds me of the "how do I tell my wife/GF she's getting fat"... I've tried hinting the gym, running, walking, not eating so much icecream....