Persistence is the key with any algae. I have rarely found any ‘easy’ option works, ie just add 20mls of chemical X and all the problems will be solved. There is one notable exception to this, and that is Flourish Excel, which I have used successfully on BBA as many others have.
I have also (partially) successfully used Flourish Excel to control the same string algae you are currently fighting. Not as the direct chemical contact that kills BBA, but by essentially ‘starving’ the algae of nutrients. Excel is an organic carbon, and dumping lots of it into your tank will encourage the proliferation of bacteria, hence the water goes cloudy. These bacteria consume a lot of nutrients (and O2 BTW), effectively starving the algae that gains it’s nutrients from the water.
The reason I said ‘partially’ is that, while the visible algae disappeared within a day or two, it came back a few weeks later, but not as vigorous. It was then a matter of fertilising the root structure of my plants, keeping up with water changes, removing excess waste etc. The small amount of string algae slowly declined and has not been visible for the last two months, and my plants are growing stronger.
Complete blackouts (no lights and tank covered in a duvet for 3-5 days) can knock this algae back, but I have tried this several times, and have found the string algae comes back just as strong. I believe this is caused by weakening of my plants each time a blackout occurs, and when the lights come back on the algae is able to respond first to the relative abundance of light and nutrients.
So, as with most algae, it is a combined approach that works, there is very rarely a ‘one shot fix’ for algae.
Hope that helps.