I have never figured out a way to rinse those strains. I mostly just don't even bother rinsing anymore.
I think Martin actually did some experiments with different chemicals to try to get those yeasts to break up after floccing out and he found some but it killed most of the yeast sooooo...
I'd say either keep/reuse the whole cake or if you're worried about flavour carryover, make a starter with a table spoon or two.