Why is my Wife's friend demanding "pokes" from me on facebook?

My wife's friend,whom I know only casually, keeps poking me on facebook.When I don't respond to these "pokes",she is sending me a message about giving her a "poke".I've tried to ignore the messages but she's sending 2 or 3 every day when I don't "poke" her back.I accepted the friend request because she's my wifes friend from aerobics at the gym.She's very persistent with this,it's making me very uncomfortable, and I'm not interested in doing anything stupid that would affect a very happy marriage & family.I'm pretty sure that this woman is married as well.I also don't want my wife getting involved in some sort of conflict with this woman.I've just started changing my workout times to avoid her as well,going at a different time with our 3 kids to avoid this woman.I don't know how well my wife knows her and I have no clue of how to even broach the subject with my wife.What do I do?