The short answer is that there should be a checkbox under the "Fates" settings called "Kill non fate attacking mob" or some such that you can enable to solve your problem.
The long answer is that this will be bad for your exp in the long term. If you setup an evac point in a sanctuary and bump your flee hp % up higher (to around 30-40%) then you will simply run away when a non fate mob attacks you. Why is this better? Because most times it takes longer to kill a non fate mob than it does to run away, lose aggro, and return, particularly above level 30. If you stop to fight non fate mobs regularly you will lose exp in the long run because you won't get enough fate credit before it ends. Even if you're being beat on by a non fate mob, it's better to focus on attacking fate mobs to increase the chances that you'll get full fate exp. Fate grinding is more statistics than behavior...you want to maximize the amount of damage that you do to fate mobs, which generally means avoiding any other behavior that impacts that value negatively. So consider your situation before you turn it on, because above level 20 or so you'll probably be losing exp.
The long answer is that this will be bad for your exp in the long term. If you setup an evac point in a sanctuary and bump your flee hp % up higher (to around 30-40%) then you will simply run away when a non fate mob attacks you. Why is this better? Because most times it takes longer to kill a non fate mob than it does to run away, lose aggro, and return, particularly above level 30. If you stop to fight non fate mobs regularly you will lose exp in the long run because you won't get enough fate credit before it ends. Even if you're being beat on by a non fate mob, it's better to focus on attacking fate mobs to increase the chances that you'll get full fate exp. Fate grinding is more statistics than behavior...you want to maximize the amount of damage that you do to fate mobs, which generally means avoiding any other behavior that impacts that value negatively. So consider your situation before you turn it on, because above level 20 or so you'll probably be losing exp.