As per https://sendgrid.com/docs/Apps/open_tracking.html
Settings may be changed through:
SMTP API (on a per message basis)
Web API (on an account wide basis)
Web Interface (on an account wide basis) // have to log into account
You didn't say exactly how you we're implementing it, so it has links to
a few different ways. Sounds like it would be easier to log into your
account and disable it there.
updated answer from comments
go to sendgrid.com/app and its one of the three options there.
with a big red disable button on it when you hover over it.