(better to use your actual domain name)
If we're doing DNS for your domain, simply remove the mail service, and add the correct MX record by hand (or have support do it for you if your plan doesn't support custom DNS management). If we were doing DNS for your domain there probably wouldn't be a problem in the first place though, since the hosting machines would see the right MX record from our nameservers....
If we're not doing DNS for your domain (note: I don't recommend this), you can disable the DNS service (and / or the mail service) for the domain so that our servers don't think they're authoritative for your domain.
You can edit / remove services from the Domains => Manage section of the web admin panel.