I don't understand what you want.
You can create an expression for a listbox so g10 no longer shows in that listbox. =if(category<> 'g10' , category).
But the value g10 still exists and will be shown in other charts if you don't make a selection in category first that doesn't include g10.
If you want to remove it completely, first select the value, then the select excluded values and then reduce the data by choosing Reduce Data in the File menu and the select Keep Possible Values. That way g10 is gone until you reload the document.
The expression given by Gilbert is correct to exclude it from the chart or table.
But you will have to include this expression in all other objects.
If category is a dimension, go to dimensions tab,
click on Category --> Edit and then write the expression
=if(category<> 'g10' , category).
Then give the Label as "Category".
This should solve your problem.