If that's the motivation, just use a non-existant TLD, or, even better, .local If the motivation is to have a killswitch, you don't want something expensive, b

If that's the motivation, just use a non-existant TLD, or, even better, .local If the motivation is to have a killswitch, you don't want something expensive, because the attackers would then have to pay for it if they want to activate it for whatever reason.