Without testing any of this.... If the host was put into maintenance mode, you could have the scope in the web alert eliminate hosts in maintenance mode. I just checked and there is field "in maintenance mode" with a yes/no value that can be setup in the scope section.
Otherwise, I would really have think about how to eliminate alerts if it is not a managed node. The usual tricks, like custom properties, would not apply.
If you only wanted to alert on hosts in specific datacenters or with a give naming convention- that would work too.
