If I am not mistaken, open worldserver.conf and set the last line to
Expansion = 0
# Expansion
# Description: Allow server to use content from expansions. Checks for expansion-related
# map files, client compatibility and class/race character creation.
# Default: 6 - (Expansion 6)
# 5 - (Expansion 5)
# 4 - (Expansion 4)
# 3 - (Expansion 3)
# 2 - (Expansion 2)
# 1 - (Expansion 1)
# 0 - (Disabled, Ignore and disable expansion content (maps, races, classes)
Expansion = 6