Slack does not have a global account system. Instead, each workspace operates independently.
Go to <http://my.slack.com/account>
Enter the email address associated with the workspace you want deactivate
Click "Sign in with email"
Slack will display a list of all the workspaces you're signed up to using that email address
Click "Show xx more workspaces" at the bottom if the list is longer than 5
Find the one you want to deactivate and click on it
Scroll down to the bottom of the page and click "Deactivate your account"
Enter your password
Click "Need to create or reset your Slack password?"
Check your email and open the "reset your password" email from Slack
Click "Choose a new password" in the email
Enter and confirm your new password
Click "Change my password"
Click on back to "Account & profile" in the left menu
Enter your password and click "Confirm password"
Click "Yes, deactivate my account"