It is even easier than that.
In the People custom Filters…
Ancestral filters >> Ancestors of <person>
Check the box to Include selected Gramps ID
If you want all the Descendants (and their Spouses) the second filter is
Descendant Filters >> Descendant family members of <filter> match
To exclude this branch, you need to find the people to exclude.
Descendant Filters >> Descendant family members of <person>
Check the box to Include selected Gramps ID if you want the selected person excluded from the final results but if you want them included, but not their family, leave the box unchecked.
For this filter you want everybody except the people in the filter, check the Return values that do not match the filter rules in the Define filter window.
And as a final filter, the filter you will actually run, you want to combine filter 2 and filter 3
General filers >> People matching the <filter> filter 2
General filers >> People matching the <filter> filter 3
In the Define filter window, leave the Option set to All rules must apply.