Restricting dimensions to used values

Hi,
I am using Power BI over an SSAS Tabular model.
We have several generic dimensions, like Nationality, which usually come from standardised data sets and have many entries.
Any particular fact table only uses a few of these.
For example there are 120 Nationalities listed but our staff come from only 20 of these.
How can I restrict the list of dimensions available in slicers to just the 20 that are used ?
This could be in either the SSAS or the Power-BI areas.

Any ideas appreciated

JC