Here are a couple of related tutorials on how you should go about converting / removing / merging etc the existing factions and their characteristics. Obviously there are many areas that need to be modified to completely remove a faction from the scripts, and I'd suggest making backups before you attempt to make any changes to the original files in case you come unstuck down the line.
Converting a Roman Faction to a Different Culture
Howto: convert Brutii & Scipii into diff factions
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