By default, both GCW and CW eras are part of a newly created map. To remove an era (such as Hoth where only GCW exists) it is a simple matter of of removing all of the following from the addme.lua (located in …\BF2_ModTools\data_ABC\addme)
era_g = 1 mode_con_g = 1 mode_ctf_g = 1
in the case of removing GCW.
If instead you wish to remove CW, changing the "g" to a "c" is all that's needed.
If modes other than con (conquest) and ctf (capture the flag), you will need to locate and remove those as well.