Discounts

Is appears that it is not possible to only allow one discount to be applied to an order.

It looks like if a discount is not Cumulative with other discounts, it is only for that type. If we have a discount that is "assigned to order subtotal" and another discount that is "assigned to categories" both discounts would be applied.

Is there a workaround to prevent multiple discounts from being applied?

Yes, as per the docs
https://docs.nopcommerce.com/en/running-your-store/promotional-tools/discounts.html

The Cumulative with other discounts option allows customers to use several discounts at the same time. If selected, this discount can be used simultaneously with other ones.

NOTE
This feature only works for discounts of the same type. Right now, discounts of distinct types are already cumulative.

So it will need to be customized.