Hi there I know how frustating it can gets. I too, used to fight cyano
for months. But they all gone away after I start using distilled water and Phosguard from Seachem.
To get rid of the cyano
in the first place I syphon the upper layer on the sand they do water change every week. Then I add in Phosguard from Seachem. Ever since the cyanos are gone.
Well I only have cyano
on the sand bed but not the rock itself so not sure how you can remove them from the rock. May be scrub them off using a brush of some kind or even throw them away!!! I think even you scrub them thoroughly they will still be some of them in the crevices where the brush can't reach. Because I heard cyano
do spread so better get rid of them once and for all just throw the whole rock out!!!
Hope that help.