Restrict users from creating issues

I run a public repo where we want to prevent non-owners from creating Issues. We want to keep it public so people can follow the development but since we have already had a spammer posting and we want to keep Issue creation to ourself to keep things the way we want, I was wondering if there is a way to prevent users from creating issues (commenting should be ok).

I see that most spam is done by users that never signed in. Maybe a solution would be to restrict users posting issues only when they signin recently?