Hi,
While this is definitely a good place for Unified Manager to tighten up the Alerting feature, another way to EXCLUDE alerting from a particular cluster or a particular object, or a particular event on a particular object, is to not to INCLUDE it in the first place.
These are two different workflows.
INCLUDE: Configuration\Alert
EXCLUDE: Configuration\Manage Events
For example, if you are getting events from a cluster that you don't want alerts, that means the cluster is already in the first list.
In order to avoid this, when you create the alert, don't use the <<All [object]>> option. Add only the clusters and objects you want alerted on. -- Again, you have to remember not to include other objects, such as node, volumes, LUNs etc.. from the list. Include or exclude cluster does not automatically include or exclude the objects underneath. This is where UM could improve.
The other way is of course what you have tried - exclude the particular cluster, object or event after it has already been included. In this case, you already know exactly what not to alert on. Use information from the alert to trace back, then shut down the flood gate.
Hope this helps (or at least did not confuse you even more :)),
Julia