In Users -> Plans section delete plan for trial sites which is marked with special color. Before deleting this plan make sure that it is not used by any of the users. It can be checked in the same section by Number of accounts column.
It can be done with special configuration parameter in <SB_ROOT>/config file. Where, by default, <SB_ROOT> - /usr/local/sitebuilder/ or /opt/sitebuilder/ on Debian servers.
Following parameter should be added into [general] section:
allow_trial_sites = off
By default allow_trial_sites = on.
After adding this parameter users will be redirected to Sitebuilder login page with "Trial sites creation is disabled" message.