RESOLUTION
Sitebuilder 2.x:In Plan Management section you need to make sure that there is no plan which will be used for trial sites. Just reset the plan which were used for trial sites and in this case access for trial sites will be denied. This will deny access for creation new trial sites and for already created trial sites.
Sitebuilder 3.x:
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.
Sitebuilder 4.x:
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:
.....
[general]
.....
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.