I don't know. I've never heard of paying by check here. Do you happen to have a paypal account with a balance? Maybe a prepaid credit card would work too. (I haven't tried.)
I just found this on Roku's site so it sounds like a prepaid mastercard or visa will work:
"MasterCard, Visa and PayPal are accepted payment methods for all Roku regions. American Express is also available for all sites except France, and Discover is available in the U.S. and Canada. Pre-paid gift cards can be used as a payment method as long as the card brand is an accepted card type, for example MasterCard or Visa."
The "fraudulent activity" makes me wonder just what payment you think you need to make. There have been scams where people receive emails or text messages stating that their Roku account would be shut down if a payment isn't made. I really hope that's not the kind of payment you think you need to make. You should only need a payment method if you have active subscriptions shown at https://my.roku.com/account/subscriptions . If you don't have any active subscriptions then you don't need to worry until you want to buy or subscribe to something through "Roku Pay".