Regarding the disable of check box

Hi everyone,

i ve created one check box in a form with reference to a serial number…

if i clicked the check box means the fields called discount amount and discount approver name gets updated…

nw once tat gets over means , the user shud not able to click tat check box again and it shoud gets disabled…

tat means one user is allowed one discount only…

tel me how to disable tat check box …

With Regards,

M.Madhan Kumar.