I have used a hand held sprayer filled with water to knock those wee buggers off plants, there is no need of a "pressure washer" type of applicator.
Perhaps this link can be of some help. Note where the article talks about Aphids and Nitrogen.

Here is a link that might be useful: About Aphids

freki, I understand that these are rather specialized aphids that don't really fit into the cookie cutter description of most other aphids.

My only suggestion is that you call your local extension office to see what they might have to suggest in late season control of Snowball aphids. Perhaps Neem oil is a possibility to give the plants some relief. Neem can be translocated by the plant and might be able to affect some of those aphids hidden deep within the leaf pockets that they create.

Oooh, good idea! the Royal Botanical Gardens (I'm in Canada) are just down the road, I'll speak to them! Thanks!

On more mature plants (the neighbour's) the little suckers aren't a threat, but on these little guys they may weaken them too much to survive the winter. That's my worry, as more than half the new growth is galled.