I have a number of clients who use the yellow jack "traps".
Blasting the nests with a power washer every couple of days also keeps them from making progress and they often "move on" to better or less hostile locations
KMS's suggestion to wash away the nests works best.
There is an native trick that works great for repelling yellow jackets. Get a small paper lunch bag and fill it with a crumpled piece of paper so it keeps it's shape. Tie a string to it and hang it from a corner of your house or tree where you want to work. Wasps are territorial and visual in nature, they mistake the paper bag for another nest and will stay away.