Dealing with Spam with an unkown party harvesting the email address

I have a situation where I keep receiving spam from several IT companies. I think I gave away my email to a company in the last couple of years for reading a white paper or something (I do not remember at all). And that company has been selling my email to various IT firms.

I reached out to a couple of companies asking them about how they were able to gather my email address but they were not willing to disclose.

What's the best (if any) way to deal with a SPAM situation like this?