You can't specify what you want an alert for all error in a certain severity level, and then exclude specific messages within that severity level. (A long long time we could do that by entering those error numbers in a specific place in the registry, but that is long gone).
I would make sure that this doesn't happen, in the first place. Why do the SQL Server throw that security, and reconfigure whatever is triggering it so it doesn't happen anymore. 25 is the most sever severity level!
If above, for some reason isn't an option, when either remove the alterint for severity level 25, or specify a rule in your mail system to disregard these emails.