Yes. Check out If Then and On Current Event

If myCheckBox Then
   myCombo.Enabled = True
Else
   myCombo.Enabled = False
End If

Kevin I think we need clarification from Masih as to what he is trying to accomplish.
The way I read this question is, perhaps, different than you.

The way I read this was that he was asking if there was a way to limit the choices within the combo box itself.
Not turn the combo box as a whole on and off.

If that is the case, my answer is NO.  You can't gray out a choice in the combo box.  
By using a query as the data source for the combo box, you could limit whether or not the staff name appears in the combo box.

Scott You are absolutely right. I read the question wrong.

Hi Masih, you need to a create a "fake combo box".  Richard has actually made a video on this topic.  Search his YouTube channel for "Can you have a List Box or Combo Box with Formatting and Colors?"  Basically, the fake combo box is a continuous form.  You set a conditional format that would disable a control when certain conditions are met.  I did an example below.  If I uncheck "show Asian countries", all Asian countries in the combo box will be disabled.  Unfortunately, disabled controls always have a grey background color.  That's a Windows system setting that cannot be easily changed.  So you would need to have a grey background for your entire combo box to make it look right.  Also, you would need much VBA coding.  Since it's a fake combo box, you have to fake its opening and closing, and you even have to create a fake dropdown button and program its behavior with VBA.

Thanks!  How can I make a fake combo box? Do you have a video on that?

Search YouTube for "Computer Learning Zone Combo Box with Formatting" to see Richard's video on fake combo boxes and list boxes.  He doesn't mention disabling a field specifically, but you can apply his method to achieve it.  Whatever formats you can use on a subform, you can do it on a fake combo box or list box.

List Box with Format and Color?
Column Colors

Thanks! I watched that video but it' all about a list box. How can I make a combo box out of a continous form?
Masih

The basic methodology in the video is the same for a combo box: you open and close it with VBA depending on user action.  The only difference is that a combo box has that little dropdown button, for which you can add a Click event.  But you'll have to write basically the same VBA code: for opening and closing the fake combo box accordingly.  Both Richard and I use a continuous form for faking a combo/list box.  So watching Richard's video should give you all the knowledge you need.