You can't delete an account in Square.  If that business runs through your personal email account, you can request them to change the email that's attached to their account.  Ideally, they would comply for no other reason than it's the right thing to do.  If they balk (depending on how many devices they have that log on to the account, it may be a hassle, since each device will need to re-log in with the new email account), there are some steps that you can take, but I don't want to get into them here since there may be legal ramifications that I can't possibly address as I'm not a lawyer.  Much of what you could do is based on who owns the account - it uses your email, but you may not be the actual owner of the account, in which case you would not have the right to make changes yourself, even if you can still log in.

BTW, if you are the legal owner of the account, the company must stop using the account and set up their own immediately, as account ownership can't be directly transferred from one person to another.  If you're the owner of record, the business's financials are still tied to you through the SS# or other information you submitted when the account was set up (this information can't be edited, which is why ownership can't be transferred.)  I can't imagine trying to untangle the tax implications if you own the account and the company continues doing business with it.

Your best bet (especially if you left the company on good terms) is to contact them and politely request them to change the email address that's associated with the account.  It's not very hard to do and any ethical company should willingly comply.

To change the email address, have them go to Settings & Account from the Dashboard then click on Personal Information.  The email address that's associated with the account is right at the top of the page and all that needs to be done is to click Change and save the changes.  Then each device will need to be re-logged in using the new email address (and password if that gets changed too - that is done directly under the email information.)

Good luck!