Per Article 8 of the EC261 directive, they must refund you if requested.
So in your e-mail, press "reply" and message [email protected] and [email protected]. Clearly say you decline a voucher and want a full refund, and that they are to respond within 7 working days.
If they refuse, that's illegal and you should report it to the national enforcement body in the country of departure.
Alternatively, you can request a chargeback with your card provider, but need to prove that you've tried to a reasonable extent to solve it with Ryanair.