How to prevent user from checking two chek boxes against same item in grid

Hi,
I have a table with following structure:
Table 1
ID = item_id
name = item_name
department_id = department_id
is_active = is_active

Table 2
ID = department_id
name = department_name
is_active = is_active
======
In grid I am showing item_name, department_name and is_active. 
I am looking for a java script function that fires on onchange event of checkbox and restrict user from making items of two same department as active.
I have done too much R&D but all in vain.

Please help.