CentOS7:

# search for roundcube config file
find / -name *roundcubemail.conf*
/usr/local/vesta/install/rhel/6/roundcube/roundcubemail.conf
/usr/local/vesta/install/rhel/7/roundcube/roundcubemail.conf
/usr/local/vesta/install/rhel/5/roundcube/roundcubemail.conf
/etc/httpd/conf.d/roundcubemail.conf

# search for installed packages
yum list installed |grep roundcube
roundcubemail.noarch

# is enough to disable roundcube
rm -rf /etc/httpd/conf.d/roundcubemail.conf



# seems to be no problem as well to completely uninstall it
# vestacp works just as well without it
yum remove roundcubemail.noarch

Debian:

VestaCP does not provide an own roundcube version, if you want to disable roundcube, you can to this for example by removing the /webmail alias from the system:

rm /etc/apache2/conf.d/roundcube.conf
service apache2 restart

if you want to re-enable it, you can create again the symlink:

ln -s /etc/roundcube/apache.conf /etc/apache2/conf.d/roundcube.conf
service apache2 restart

This will work for Ubuntu and Debian, should be a similar way for CentOS/Redhat systems. (src)