Star polyps are the Devil! I wish I had never introduced them to my tank, unfortunately I new I would regret it due to the research I did....was I ever right! They were no problem for 6 months or so and then BAM...population explosion...they grew faster than I could eliminate them and stung/killed some of my prized pieces....I will never again allow a Star Polyp in any of my tanks...
BTW, I too have tried to eradicate them, mostly through scrubbing, harassment, etc. The only effective treatment I have found is long term exposure to air. Though I do believe short term concentration of intense light could also work.
Chris