Firstly you need to set up a blank Security group - Go to Admin/Security/New Group to do this.
If you want to play it extra safe, untick all the boxes on the left. The crucial point is not to add any of your users into the Group Members field in the middle
Now you need to go the Business rule itself (Config/Business Rules/User Defined Rules) open it up...
Now tick "Apply Rule to Selected Security Groups Only" and choose the security group you created above.
In effect this applies the rules to no actual real users, but as far as RE is concerned there is a user group that the rule is applied to so it's happy.