So-called ignore rules can only be used if they apply to a single host. Applying them to a wildcard match against the name or a tag is not possible. So make sure you have selected a single host as the first condition.
Let's say the default rule
ALERT: Agent - File system free percent (any disk) Last value is < 5 % is suitable for all your host except one. How to create an exception without deleting the default rule?
Create the same rule again but apply it to a single host with the "Host is" option. Select "Ignore" as action activate the exit option. This will stop processing rules of the same type for a specific host. So for this host the default rule will be skipped.
Make sure the exception rule is placed before the default rule. Use drag and drop to change the order of rules.