Thanks for reporting this! It's a glitch with the code that makes sure you don't snowball a blocked user.

A fix is being worked on now. You can continue using the Random option in the meantime; you are not at any risk of getting in trouble here.

[edit] And it should be fixed now. Well not completely fixed, but the error message you get is less scary and doesn't make it sound like your fault anymore.

(Basically: the original error message was part of a change that was made after some users pointed out you could accidentally snowball someone you blocked via the random option. An update was made to prevent that, but we overlooked that the default error message for trying to interact with a blocked user is kind of.. threatening. Ideally we'd just have it not randomly choose blocked users, but that's a bit complicated for reasons I won't get into here, so for now I've changed this to have a nicer message in the chance it happens again.)