Can I still use Vouchers to offer a specific discount to e.g. a returning guest?

Yes, if you do wish to offer a specific discount to a returning loyal guest, or for any other reason, you can generate a voucher in SuperControl to the discount value or percentage that you would like to offer. The guest can then redeem the voucher during the booking process as normal.
This will allow you to balance profitability with loyalty, and ensure that you remain in ultimate control.