Exclude results in a visual

You can create a filter for the visual.

If you need a card with the same filtering, you can set a filter to a card or just copy/paste the table and set it as a card instead.

You could also set another visual level filter to say only where severity is high to narrow it down further (when you add more data).

Hi Tribunal,

Here is my solution:

Count High = 
CALCULATE(
    COUNTROWS(T_Severity),
    FILTER(
        ALL(T_Severity),
        T_Severity[Severity]="High" && LEFT(T_Severity[DeviceName],4) <> "xyz-"
    )
)

Create the card.  Drag a field on to the card and select your aggregation (or create your own measure and put that on).

Drag DeviceName into the Filters pane where it says 'Add data fields here'. Select Advanced Filtering and 'does not start with', put xyz in here