Zelle app: uninstall it. If your bank offers Zelle, you do Zelle through your bank.
Bank app: you can uninstall it.
Zelle enrollment: contact your bank and tell them you want your Zelle deactivated if you don't wish to use it. They will reach out to Zelle to get you unenrolled. You cannot use Zelle without being logged into your bank in some way. Obviously, keeping your passwords safe, clearing internet history, not using bookmarks for financial institutions are up there with keeping your accounts safe.