Just trolls being trolls.
I wish they had included a report for "Exploiting" like they did on MW3, as that is basically what they are doing, exploiting a feature of the game to ruin the experience for others.
Use the XBL reporting tools to avoid them and report them and leave the lobby.
See this from time to time in HC TDM, but not as bad.
Sad that people have "fun" by ruining the fun of others.
I've had the blocking thing done to me a few times.
I figure I'm easy bait for that as I use a LMG and try to hunker down in a corner to reload when possible.
Like drew said, just use the XBL report feature so that you can avoid being matched up with them in the future.