We already effectively pay for a Zwift Licence, it is 12.99 GBP / month in the UK.
A possible solution to the weight/height doping would be that every 6 months you have to get a weight / height verification done by a Zwift representative, someone who is local to you.
Part of the Zwift Licence fee could be used to pay an official Zwift Rep for this purpose.
Some price restructuring may be required, such as you pay for 6 months at a time. That is up to Zwift to decide.
If you do not or cannot get your Licence approved by attending a “official Zwift weigh in”, you would still be able to use the app but no official results would be recorded in KoMs or races.