"your group is neither welcome nor invited to attend"

"Thanks but no thanks" might have been a rather better tone to adopt. In an email.

If the winter goes on the way it seems to have started I can't see myself braving the icy fens. Maybe now I won't mind that so much. It all sounds a bit sour - "we are involved in delicate negotiations...Any actions on the day which are not built into our program will be seen as a lack of responsibility..."