Treating suckers would kill the plant. Cutting them back is the best way to manage them. You can also dig them up, cut them off , and transplant them to propagate the rose elsewhere on your property or give the plants away. But there really is nothing to do about suckers. 

Is it true that only grafted roses grow suckers, and how do you know if the roses you’re buying are grafted or not?  Thanks!

"Suckers" is a term generally used for any stems that emerge away from the main plant. They are formed from buds on underground roots growing out away from the plant and is just a way a plant grows to a larger size. 'William Baffin' is on its own root, so it's not grafted.The suckers you see are just the way the plant is spreading and expanding its size.

Suckers also grow from the rootstock used when grafting plants - suckers are common in crabapples. A graft is made by cutting and joining a rootstock from one plant to the scion (a stem cutting from one plant). Rootstock can be selected for plant hardiness, disease resistance and plant size - dwarf apple trees for example. The scion is selected from a compatible plant for flower color, fruit quality, foliage. As the graft heals and the two plant parts grow together, they form a callous that can be kind of knobby and usually easily seen. If a graft is buried (i.e. planted too deeply), the root stock will sprout.