Ailanthus altissma is a fast-growing invasive tree in Washington. It's very hard to control once it establishes its taproot. The best control can be effected when the trees are seedlings. Pull them out whenever you see them. You'll also want to check frequently during the year and remove root and stump resprouts as well as new seedlings. If you have a lot of these trees, prioritize and remove the female trees first (the ones with the flowers). Herbicide treatments with glyphosate (E.g., Roundup) and tryclopir (E.g., Garlon 4, Crossbow) have had limited success. Biological control for Ailanthus is not yet available.
The bottom line is that there is no easy way to control them. Be vigilant throughout the year, pulling new seedlings and cutting stump regrowth. Here is some more information discussing the difficulty in controlling this tree. http://www.oregon.gov/ODA/shared/Documents/Publications/Weeds/PlantPestRiskAssessmentTreeofHeaven201...