no matter what the game when it comes to online there's always the posssibilty of ''host booting'' which in simple terms is flooding an ip address that your in contact with using ''bots'' which is a collection of viruses that hackers spread on random peoples comuters through torrent downloads which allows the said hacker too use these bots (peoples computers) to flood the ip address of the victim and the more the hacker has spread them the more bots he has in his arsenal. so in heads up you should only have to ip's yours and the opponents which you can get by using a ip sniffer which is widely available. Whether or not this is known and counteracted by the sites is unknown but i too have been playing heads-up and have had my internet interfered with but i quickly reset my router and rejoined and still won

but the moral of the story is that it is possible to do this as it has been happening on xbox live for many years and people have been doing this to close small web sites connection as well and its not the hardest thing to do so its quite common :s
By the way I'm by no means trying to condone this action or trying to scaremonger I'm simply sharing my experience in this pathetic cyber crime