Congratulations! Having cracked versions out there indicates that your software is both useful and notable, which is a point where many developers would
love to arrive at, maybe, one day.
As to what to do about it, there are two basic tracks, neither of which is incompatible with the other:
1. The legal route: notifying distribution sites that they're hosting content in violation of copyright law, requesting takedown, and if required, demanding such (with a DMCA notice or similar). If a distributor is unresponsive to both, DMCA Google, Bing, etc. to at least suppress the cracks in most search results;
2. Having some fun with it: make sure that a special page on your own site becomes the #1 search engine result for " crack" and similar. On this page, put a human face on your pricing, explain the available discounts, and finally offer to supply a free registration key to anyone that truly wants it, so they don't have to download all kinds of shady binaries. Of course, any such free keys don't come with support and display a prominent PIRATED VERSION notice in all window captions, printed outputs, et cetera, but you disclose all of that upfront and explain how that's a small price to pay...
But above all, make sure not to confuse a "pirated copy" with a "lost sale". In 99% of cases, the Venn diagram of those has very little overlapping area.