Use an alias.
If you run your own email server then this is simple, otherwise check your email provider if they allow alias'.
For example, instead of merely $FirstName.$LastName(at)Domain.com, create an alias of $FN.$LN.$SiteName(at)Domain.com so that you know *exactly* where you used that alias. Now, if you get spam to that alias you know *EXACTLY* who leaked/sold/spaffed it to the spammers. You can then either delete that alias (so your provider no longer accepts email to it), or write an email rule to auto-flush/forward said alias. It's rather satisfying to auto forward the alias to the site that spaffed it, addressed to their CxO's, CC'd to their HR/Legal/Marketing departments, along with a note as to why they can continue to enjoy the sudden flood of spam. YOU never see that alias again, THEY get to deal with the spam.
You can then go hit the site that screwed you over, remove any/all of your PII (assuming you gave them any), try to kill the account, & then wash your hands of them. Your account now claims you're a 98 year old, underage, single, divorced, childless, 158 kids, zero income, $1M+ per year, turtle humping, lawn mower snogging, beer snorting, coke drinking, $PoliticalParty voting, inbred, sister marrying, $DenominationChurch going, transgendered lesbian hermaphrodite from London named DonaldMulla TrumpMohommed or something equally as bogus. Screw up their data, fuck with their heads, & auto-perma-delete anything to the email alias you used with them.
I've done it here, my bank, & everywhere else that I've had to register an email address at. That way I have *proof* that spam to any alias HAD to come from $AliasedSite & not anywhere else, since the *only* place that alias ever got used was at that *one site*. Their legal department can't wriggle out of the proof staring them in the face when I show them the FN.LN.$URL email address that suddenly starts getting flooded with spam. "Sorry but the ONLY place that email address got used was with *you*. You HAVE to be the source of that address, it doesn't exist anywhere else but in *YOUR* servers. So have fun dealing with the spam it gets, & delete my account before I decide to press charges. Have a nice day!"
*Gleefull rude gesture*
Alias. Use them. Everywhere.