Don't offer to void a refund

app/models/spree/payment_method/taler.rb

@@ -26,7 +26,9 @@ module Spree
       end
 
       def can_void?(payment)
-        payment.state == "completed"
+        # The source can be another payment. Then this is an offset payment
+        # like a credit record. We can't void a refund.
+        payment.source == self && payment.state == "completed"
       end
 
       def can_credit?(payment)

spec/system/admin/payments_taler_spec.rb

@@ -76,8 +76,7 @@ RSpec.describe "Admin -> Order -> Payments" do
 
     # Our payment system creates a new payment to show the credit.
     within row_containing("$-9.75") do
-      # TODO: don't offer to void a credited amount.
-      expect(page).to have_link "Void"
+      expect(page).not_to have_link "Void"
     end
   end
 end