Root fragments may remain viable in the soil for up to 2 years. In addition the seeds are spread by water and wind, as well as with contaminated crop seed, feed and manure. Deeply buried, Canada thistle seed can remain viable in the soil for years. All the weedy thistles mentioned above are invasive non-native plants, originally from the Old World. There are also several species of native thistle in the Pacific Northwest, but these are generally not invasive or weedy.
Prevention is the best line of defense.
You can plant bare areas with grass or other groundcover so that seeds blowing in have no place to get established. Or you can dig out your biennial thistles before they flower. Since biennials do not reproduce from their roots, if you sever the root from the plant below the soil surface, it will kill the plant. To deplete the roots of stored food so that the plants will finally die, all leaves must be removed within four weeks of emergence. Leaves remaining longer than this can replenish the plant's food reserves and the gardener is back where they started from.