I generally try to find the true email address of the person who sent me to invitation and then simulate an attempt to visti LinkedIn while GNU LibreJS is enabled and register the results of non-free JavaScript in the email message telling to the destination how LinkedIn forces people to use non-free JavaScript.

Besides, I tell them how bad is the network desing of LinkedIn, since it's centralized and so it's users are susceptible to terms of (ab)use, censorship, and also, the risk of being unable to interact with Linkedin from outside (since it's not decentralized, be it either federated or distributed, there's no way to interact fully from outside), that is, either you use LinkedIn, or you won't be able to interact "in the LinkedIn way".

Since I started doing this, the spams are gone. :D

Oh my... And there are speakers who talk good things of this centralized curriculum vitae eater. I have even seen free software supporters talking good things of this crap... Ugh..