You can remove all the subscribed SD Topups via Airtel Selfcare Portal Online
HD Topup link removed from Airtel Selfcare portal from last Month (June 1st) and I have no idea when it'll be back but when it's back HD Topups too could be removed from Selfcare portal itself.
Removing by Calling Customer Care is always an option and if you want topups to be removed via sms there were multiple formats available for different Topups
If it's a Single Channel
REM <channel number> to 54325 or
REM <Channel Name> to 54325