This is a pretty good example of why I find being very blunt and a little rude a much more effective method of getting someone to leave you alone.
I joined a lobby and played a few games with the same set of players on both teams. The guy who sent me the following messages was on the opposing team (with his friends) the whole time. He sent me between 7-10 private party invites- I declined them all, at which point he started sending me text messages (and continued to invite me to a private party), eventually moving to sending me a friend request, which I rejected too.
He sent me a friend request at this point:
Some people just can’t take a hint.