First, people or companies using the cracked version of your software were never going to buy it. These people do not pay for software, and nothing you do will convince them to pay for it.
Secondly, whatever measures you put in place to combat the cracked versions will potentially have a negative impact on your actual paying users - the real people you built the software for.
Extra security measures or hoops to jump through; extra validation or 'calls home.' - all extra things that can and will go wrong at some point for a real user.
Why would you inconvenience paying users to combat people who will never be paying users?
Ignore them, and spend your time improving your product for your real users.
[I've had 3 desktop apps cracked and hosted on Warez sites over the past 15 years - even appearing for sale on eBay on some occasions. I was worried the first time it happened, then I stopped caring. They made no difference.]