Remove anything bigger than gunboats from vendors.
Remove anything bigger than gunboats from players, turn them into weaponless starfliers, with of course compensation paid.
Add shipyard modules to player bases, accessible at core level 4, taking up 2-3 slots.
Add resource requirements for all kinds of vessels of the same size or bigger than gunboats.
Add scrapping ability to shipyards: 40% of all resources can be returned upon ship change.
Add ability to set faction shipline manually (got no clue by which could the system check for an appropriate one). Given the few bases, admins could check those, moreover, make an extended sheet of player bases indicating location(system), core level and/or presence of shipyard, accessible only for them. This is to filter unreasonable tech line choices. (No turtles being produced on a shipyard deep within Liberty.)