How can I disable this option for guest?

I already tried in group permission but it still appears

Disable POST BEFORE REGISTERING in your Admin CP.

It's actually a really good feature to increase conversion of guests to community members. You might want to take a look at the feature overview here-

 

Do you want to simply stop them from creating threads or the ability to post altogether?

Regardless, you can set either and/or both permissions - ACP > Members; Groups > Guests > Permissions > POST NEW TOPICS; REPLY TO TOPICS [Toggle as desired] >Save