You've got two, possibly three, as humble ol' Whacker sees it.

1. Box of 8 trick. See thread here for description: https://forums.totalwar.org/vb/showp...8&postcount=14

2. Nerf them in the xml and txt files that you can extract using the unpacker. You can reduce their changes to succeed, amount spawned, move rates, etc... Search the modding forums for help on this.

3. Use your (useless) assassins on them. I say useless because I find the base unmodified assassin's chances to be horrible, so I bumped up their base chance to succeed and now they work like I'd expect them to.

Good luck.