The starfish only has access to about 20% of the sand surface as he only ever stays along the front. I have never seen him travel into any of the caves or spaces in the rockwork or along the back of the tank. As for the 6 line, wel i would live to have him out, but impossible without a teardown wich is not happening.
Having said all that, this is a recent problem and suddenproblem in a mature tank, i have had all of these creatures for a long time so if they were wiping out the SB i would think that it would have happened along time ago when the tank was new and the SB was not yet established.