There's no way to "clean" them. Trust, I understand the desire, but I'm not sure using pirated plugins on production e-commerce sites is the best idea. If your website gets malware from it, your customer data is at risk along with the business itself.

Some plugins have simple variables to check if you purchased, and you might be able to change it in the plugin editor and get the full features. This won't work for plugins that phone home.

Very bad idea.

Theoretically you shouldn't need to, who buys a plugin is entitled to its source code because it must be licensed under GPLv2 or later due to Wordpress's own licensing.
Despite that, I haven't dug much, but I am surprised I haven't found an alternate store that redistributes plain plugins without the paywall.
Are you sure you need the paid-for plugins, or could you get away with free (gratis) ones?