Hi there,
I agree with Sreekanth (+5)
In CUCM look up the MW-Off number (Feature>Voicemail>Message Waiting>Directory Number>Message Waiting Indicator Off )
This will be a Directory Number like perhaps 2600. Go to the phone, press the Line button, dial 2600 and the MWI Lamp/Icon will turn off.
Or if you need to do this via remote then you can temporarily re-create a mailbox for the DN in question. Get it set up, send a test message, listen to and delete the message, then wait a minute before deleting the mailbox and you should be good to go.
Cheers!
Rob