are you talking about nulled themes? or if you buy one?They probably have some PHP or javascript tracking code tucked away in the theme somewhere. If its javascript it would show up in chromes developer tools when you load the site, if its php you would have to locate it in the code somewhere and remove it.
I think some have those codes some don't. Only way is to search it
By purchasing themes on themeforest for $40-$50 you'll get a regular license, and you can use it only on 1 site.
If you want to use it on unlimited number of domains - you must purchase Extended license - $2k!
Theme authors are using different ways to protect their themes from being illegally used. Some of them add js code that will let them inject more code on sites that are stealing theme. So, beware
Personally, it's way better to purchase themes at some premium shops, where for a price of $40, you get an access to 80+ themes etc, and you can use it on unlimited number of domains.
i personally download premium wordpress themes for free and use them on my project
if you want to use another service check digishops .xyz
There is a simple solution man, contact the developer offer him a few bucks to remove restriction on theme and code
so you can have complete access to it and do whatever you want, let me know