I used erythromycin for cyanobacteria growth reduction, but it was in higher plants culture. Maybe you may find some other antibiotic for your purposes. Or try to isolate an axenic culture, and after that keep your algae under aseptic conditions.
I'd plan to re-start the Haematococcus culture -- start with Haematococcus cysts, centrifuge/rinse several cycles with fresh media, then micropipette single Haematococcus cysts into replicate containers of fresh medium.