Wireless Internet Usage

Dear All
We are into the shopping mall industry, inside the shopping mall we are giving wireless internet fascility,and we will charge the users depends on their usage.
Here it is happening like
1. the user will request us for the permission for internet usage
2.We will grand them permission
3.At the end we will charge them

Questions ?
1.How i can i restrict the users/how i can grand permissions [ thru any password / any key ]