[MM3-users] Disable signup-requests for mailman3 via e-mail

Hi Mailman-Experts,

how can i completely disable/block list-signup-requests for specific lists via e-mail? i have some announce-only mailinglists i do not want to allow users to signup at all via e-mail. I'm aware of the subscription policies but there is no block or reject/discard option.

Thank you.

Stefan