Hi mkirchner,
Good news! Yes, it most certainly is possible to stop rebel spawn rates, both technically, and non-technically.
In game to make sure your rebels don't appear you can make sure that your loyalty is above 100% - this often prevents any filthy rebels spawning at all. Also, having Unrest in provinces, this is debated by some, increases the rebel spawn rate, so it would be best to remove those horrible Unrest causing governors. As a final note, as you have said, Rebels like to appear out of your line of site, so situating Watchtowers, although this is debated, or Forts in a province can reduce their spawn rate and manipulate where they are spawned.
Ouside of the game, in modding terms, you can prevent rebels from appearing by going into either "C:\Program Files\The Creative Assembly\Rome - Total War\data\world\maps\campaign\imperial_campaign\descr_strat.txt", if you have Gold, or "C:\Program Files\Activision\Rome - Total War\data\world\maps\campaign\imperial_campaign\descr_strat.txt" if you have a standard edition, not Gold. On line thirty-six/thirty seven change the spawn rates for Brigands/Pirates to your personal preference. I, think, the higher the number the lower the rate, but I'm not too sure on that one, sorry.
Hope this helps, cheers!
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