Felip's solution should work but I will create another table with valid regions in the script.
The reason is that even you have define a title for a filter:
When users make some selections, the current selection on the top will not show the filter title but the if clause, which is messy:
I will create a table in the script like this:
load * Inline [
North America, North America
South America, South America
And use the Region as the filter in the front (the current field is region, which has the values you don't want to show).