I never had register on exchange.bitcoin.com but I received a mail to complete registration there. It has been more than one year when I last visited bitcoin.com, I even didn't knew about the subdomain exchange.bitcoin.com, this is very stupid way to reach people. I am surprise where they got my mail, I have used it in bitcointalk and some other places which are non crypto website.
It should have an
"Unsubscribe" option.
If it still spams, then there's something
"unusual" happening and it needs to be reported.
But it really weird that you have become an email subscriber without your permission.