Just tell her you do not think it is a good idea, based on past incidents.
AND maybe she is contacting you again for play-dates, because, NO ONE wants to get together with her son.
You do not have to get together with her to "explain."
Nor would I want to continue a friendship with her.
It is not about being "nasty" or not. It is just having common sense.
You do what is best, for yourself and your son.
You do not owe her anything.
Put it this way: I used to babysit a Toddler, that used to do that to my daughter, who was also the same age at the time.
And I.... quit, babysitting that girl. I nicely and fully explained to her parents what was going on, showed them the scratches and bite marks on my own daughter... and my daughter would try and defend herself. I saw it with my own eyes. My daughter was miserable, though she tried to be nice to that girl whom I babysat. BUT.... I quit... babysitting that girl. With no apologies. That girl and mine, did NOT get along. And the parents though, did admit that their daughter was very.... "bossy" and a jealous type.