I have had a few isolated, tiny tufts of BBA, probably because I've been experimenting to find the right level of CO₂ and light.
I have found that 3% hydrogen peroxide (H₂O₂) is fairly effective; turn the pumps off for a few minutes to reduce water movement, then use a syringe to very slowly dribble a few millilitres onto each tuft - you can safely go up to 1 ml per gallon of tank water in total. Bear in mind that the H₂O₂ is heavier than water and will sink, so dribble it just above the tuft of BBA. Then leave the pumps off for a further 10-15 minutes to avoid dispersing the H₂O₂ while it bubbles away. Sometimes a second treatment the next day is required. The BBA will change colour, going slightly reddish - then it'll disappear over the course of a few days.
I'm sure once I stop adjusting things the small BBA problem will go away, but in the meantime the H₂O₂ works as a spot treatment for the small tufts I get.