Let's say you're signing up for a new gym membership, but you don't want to give them your main email. So add a plus sign to it: your.name+GYM@gmail.com. (Anything you put after the "+" will work.)

When the first email from the gym shows up, click on it, then click "More," then "Filter messages like these." A pop-up will appear, and enter the plus sign address into the "To" line. Click "Create filter with this search."

Then you get to choose what you want to do with those emails. Want them all to go directly to trash? Just check "Delete it," and hit "Create filter" to save your setting. And you'll never see an email from your gym again! They'll automatically all go straight into the garbage.