I guess you are talking about optional prompts (Combo seems to be VB terminology).
You cannot disable a prompt, but you could make it optional.
Create the following self join at your universe. By self join, just right click on the active panel of the Universe, choose join and paste your code.
(Table1.Country IN @Prompt('Select Country or
enter * for all Countries','a','Class1\Country',multi,free) ) OR
('*' in @Prompt('Select Country or enter * for all Countries','a','Class1\Country',multi,free))
What this code does is, it allows you to select a country from the LOVs. If the user enters “*” instead, it will return all the countries.
The part
(’*’ in @Prompt(‘Select Country or enter * for all Countries’,‘a’,‘Class1\Country’,multi,free)
becomes * in * and hence all the countries are returned.
If you are using WebI XI R2 for your reporting, you could extend this syntax as
(Table1.Country IN @Prompt('Select Country or
enter * for all Countries','a','Class1\Country',multi,free,Not_Persistent,{'*'},User:1,) ) OR
('*' in @Prompt('Select Country or enter * for all Countries','a','Class1\Country',multi,free,Not_Persistent,{'*'},User:1,))
{’*’} - Defaults the prompt value to *
Not_Persistent - makes sure that the default value is displlayed each time the reports is run. the last chosen value is NOT PERSISTENT.
User:1 - order of display of the prompt.
You could refer to the following links for more details.