Check box got locked in Access Form

Hi I've created a data filter form in my access program. I have put 5 check boxes in the form which named as district (i.e. BK, SK, ARV, GN & NMD) now I've bound value to the check box as under, (note my first checkbox name is "Check0" and added code to after update event)

Private Sub Check0_AfterUpdate()
If Me.Check0.Enabled = True Then
Check0.Value = "BK"
Else
Check0.Value = ""


Me.Check0.Enabled = False
End If

what I have to do is if I select checkbox 1 then district will be "BK", if i select check box number two then district will be "SK" and so on.. I've to make data filter on basis of check box.

But the main problem happens here is "THE CHECK BOX GOT BLACK SQUERE AND I AM NOT ABLE TO UNCHECK THE CHECKBOX"

kindly help me how can i assign a value to a checkbox directly for filter data

thanks