For example, there are two insurances: Insurance A and Insurance B
Insurance B is superlative to ensurance A, this means that if you select insurance B, there's no sense on selecting A as well since it would lead to an overprice.
Select Additionals
Click on "New"
Complete accordingly
Here's a complete guide on how to fill every item
https://help.rentlysoft.com/en/articles/5391251-how-to-load-additionalsSelect the additional you wish to exclude
By choosing this other additional/insurance it would mean that if you select A, you can not select B and the other way around.