Depends on the: brushes and your location.

The internet does not know what roots you want to kill, nor what methods are allowed in your area.

Keep your money local, go to a local store, nursery, garden supply, hardware store. Bring a sample of the brushe(s).

The most efficient and environmentally sound approach to deal with this is to cut the plants off at their base then dig up the roots. No chemicals required, only elbow grease, and if you are thorough, pretty much a 100% removal.

BTW, the plants you are referring to are shrubs or bushes. Brushes are tools used to scrub things with :)