Hi Niru,
I'm afraid this is not done through Enhancement. There is a clear & standard way of doing this through Customization.
Steps
1. Tcode OIAL
2. Click on Influencing button
3. Double click on Notification Type.
4. Give your Notification type (Say M1) where you want this field to be disabled.
5. Press Enter
6. Give the following setting. (Select Disp Radio button for field PRIOK)
See the result
Jogeswara Rao K