One user can select many values from the combo box, only thing is when the user exits the form and again re-open, the selected values of the combo box should be greyd out or disabled. And again if the user wants to select some values for the combo box, they should not be greyd out as soon as the user selects them. Hope i am clear here. Thanks.
That was just as clear the first time ... as was the answer.
It can't be done. Combos are either enabled or disabled - all or nothing.
If you are using a multi valued combo, the rules are a bit different but the above is still true
MVFs are generally considered a bad idea & will cause you lots of problems if you continue using them
For an explanation, see this article on my website
http://www.mendipdatasystems.co.uk/multivalued-fields/4594468763
For non multivalued fields, what you could do is modify the row source after each selection is made to remove that from the list and continue if appropriate till all have been selected.
However. an easier way of doing that would be to use a multiselect listbox so you can see what has been selected each time. However on reopening the form, by default the listy would be cleared so you would need to add code to recall previous selections and re-select them
Either way, it needs a fair bit of code.
The fact that this is not available as a standard feature is that it doesn't fit 'standard' database use.
It may be that your design requirements need modifying.
Good luck with finding the best solution for your needs