Basically, you shouldn't, as it's the brain's way of coping with the anxiety correctly.
Above all, DON'T use beta-blockers. In spite of what the medical profession says, that's crass. It's crass because beta blockers stay in your system 24 hrs a day until they're used up, but your anxiety attacks only last a few minutes.
For the rest of the 24 hrs the side-effects of beat blockers make you feel hideous, -tired, depressed, bored, lonely, utterly wretched.
Worst of all, to compensate for your pulse being slower than it should be, your pulse becomes actually STRONGER, because your heart beats harder to maintain (well, restore is a better word) the cardiac output -i.e., blood flow rate. Doctors should know this, but mostly they don't.
Taking deep breaths will help, by increasing the amount of oxygen in the blood flowing, so reducing the need for the heart to beat so fast.
Relax, close your eyes, in a darkened room if you can, -and breathe deeply.... it's all you can do safely.