i dont think you can respond that you dont want her going to the party. I would maybe respond more along the lines of stating what you and the kids are doing that night.
be very concious of how you come off to her when telling her this. No guilt, anger, but act as if you dont care.

regarding her inviting herself to the other activity, whatever, that is fine. immerse yourself into the kids.

M 37
W 34

T 12
M 8
D 7
S 4

Need break 4/12/15
W no ring 7/7/15

Separate room 4/12/15
Separate living suggested 8/15
W moved out 11/1/15