Yes it is possible. Just simply call CC & ask for disconnection, if they do not entertain ur request, directly email to 121@in.airtel.com
Also while mailing, correctly mention your land line no. in Subject of mail.
It depends on what registration/installation amount you have paid & no outstanding amount should be there.
They will give u a stipulated time & then it will be disconnected(Usually 7 days from date of request).
PS: They(Airtel BB guys) will try hard to retain you by offering some lucrative offers but those offers are only valid for next 3 months.
Make sure get your confirmation and clear your dues by visiting the nearest Airtel relationship center .. coz they basthorses have a nasty habbit of keeping you billed even when you disconnect and then follows the usual harassment.