Yes!

After each beta reading job is complete, you’re encouraged to rate the beta readers that participated via the My Beta Readers link on your dashboard (which only shows up after you have run at least one beta book through the system as you won’t have any readers to rate ahead of that). 

From that same screen, you have the ability to either block a reader from being able to sign up for any of your future beta books, or request that a reader be selected for any of your future beta books (provided that that reader signs up for it).

These flags can be changed or removed at any point, but must be set at the time your sign up email gets sent to readers for it to be applied.