Hi, Sarah - 

Unfortunately there are not a lot of options other then to manually remove them.

Suckers should be removed as soon as they appear. Once suckers start developing on a tree they will usually continue to occur for the rest of that tree’s life and will need to be removed regularly. Suckers that grow up in lawns are easy to manage with a lawn mower; those that grow from the base of the trunk or from roots in landscape beds will need to be removed by hand. Ideally, they should be cut back to the point where they emerge from a root or the stem, as leaving a stub can make the problem worse by causing multiple shoots to form. Removing suckers that arise from roots may require a little digging.

https://yardandgarden.extension.iastate.edu/how-to/managing-suckers-fruit-ornamental-and-shade-trees

Good luck! Hope this helps.