Wrap HasRight method of RT::Principal class. This custom HasRight denies CreateTicket and ReplyToTicket rights to the guest user. Name of the guest is defined by $WebPublicUser config option. Read more about that in RT::BugTracker::Public's doc.
To install RT::Extension::rt_cpan_org, copy and paste the appropriate command in to your terminal.
cpanm
cpanm RT::Extension::rt_cpan_org
CPAN shell
perl -MCPAN -e shell install RT::Extension::rt_cpan_org
For more information on module installation, please visit the detailed CPAN module installation guide.