TurboTax support showed me how to access my linked accounts in order to remove them.
Sign into your account > Select "Documents" in the left navigation menu > "Linked Accounts" tab > Select the options button for each account (3 vertical dots) > "Unlink {account name}"
I'm not sure the account data has been fully removed because the Investor Center view still shows that I've hit capacity even though almost all of my accounts have been removed.
