Beta blockers supress the heart rate. I know this from experience. My training level has dropped from the 120 bpm range to about 90 at the same exercise level. I am on Sotalol for Afib. However, as SassyLassie says, unless you are suffering from tachycardia, there is no problem with increased heart rate during exercise; that is what is supposed to happen and you wouldn't want to medicate to prevent it.
What is behind your question?
Well, it is normal for it to rise some during exercise, that is why the docs tell you to do it. If it races, then that is a different story, but to just supress, that would not be a good idea, it is healthy for it to elevate during exercise.