To control what alerts effect what device is based on how you build your alert.
For example I have a custom property I assign to every device called CustomerName. So when I build my alert I be sure to set that has one of the condition for the alert that the device must contain the CustomerName is picked. So let me show you few examples.
Here is alart I use to watch every interface on my network. I could filter it more by like device type but i choose not to.
Here is an alert I use to watch all my apps running on nodes based on my CustomerName.
I hope this helps you out.