How do I best remove stones mortared in around my outdoor spa & pool?

Sledge hammer and take out all your aggression on it.

Rik,

Do you want to completely remove the mortar bed the stones are set in? If so, rent a light duty jack hammer. It will make short work of the mortared stone, which will come up fairly easily. if it is just a small area and you have a reciprocating saw, buy a chisel blade for it and go at it with that. And there is always the ol' club hammer and cold chisel if you want to get funky with it.

HI Rik,

Seth said it very well, it could be a labor intensive job to remove this depending on how you want to go about it. The sledge hammer might be a bit rough on the joints.