I think if you really don't want to be friends with Mary then don't be. Why should you go through all that if you don't even like her? But have you ever though that Mary might be following Kate etc to be friendly with you again? You have been friends for 9 years and maybe since you've made friends with Kate, Mary has got a bit jelous and feels left out?
If you are going to leave her though, I would do it nicely. You've been friends for so long and although you may hate her right now, in the long run you might miss her and want to speak to her again. Don't gang up on her in a nasty way.
Also just to add... over time this will simmer down :) x