If you're using a smart trainer and don't have an actual cadence sensor paired, you may see estimated/virtual cadence show up in your .fit file post-ride. We will be fixing this issue in a future update.
While the ideal scenario would be to use an actual cadence sensor, you can also strip the cadence from your .fit file prior to uploading it to a third-party site by using a .fit file modification program. There are a number of both free and paid .fit file modification programs and websites that allow you to do so.
There is not a way to remove this data prior to its upload to my.zwift.com, and we apologize for the inconvenience.