There is no practical solution other than banning this activity. Reason is the lack of awareness among the people that they could celebrate some other way. I am not an expert in kite flying but the thing I know is that... normally you need just 10 or let us say 100 people around you to enjoy actually, not 10,000s of them.
I have no statistics as to how many people actually celebrate this activity, but if you have awareness, government can allocate (say 10) parks in Lahore for a 1000-2000 people to celebrate this activity. I am pretty sure we Pakistanis are very much capable of creating the atmosphere of roof-top anywhere, be it a green grass of park. Since number of people interested would certainly be many times the combined capacity of these parks, so you divide people in weeks (and parks) starting from 1st February to 31 March or may be even later.
I have no idea how to stop people from reaching these parks every week :) .