Sargassum's not so bad .. at least I think it has a neat look to it. It looks wicked in a refugium, like a miniature/bonsai kelp forest.
But it does just tend to grow and grow. You can pull it out though. It's not like caulerpa that pretty much needs armageddon to completely eradicate it. If you remove the holdfast of sargassum (the root like part that attaches to the rock) then it's pretty much gone forever. If you want to keep it you can just cut it down, as long as you leave the bottom part it might come back.