How can I set it so normal registered members can only give out positive and neutral rep? Basically deactivate negative rep for only the registered members group?
As far as I know, you will need a plugin to specify which groups cannot give out negative reps, as this functionality does not exist in a standard MyBB installation.
To change the settings for the entire board and all the groups, you can go to:
Admin CP > Settings > Reputation > Choose your reputation options here
I'll look for the plugin! Thanks.