Then anyone who has access to the hosted site has access to the keys and hence the data.

If you're using SQL column-level encryption then, unless you're encrypting by password, the keys are stored in the database and sysadmins have full permissions on those keys and hence to the encrypted data.

The only real way to prevent a sysadmin from accessing data is to encrypt it in the app and store the keys somewhere where the sysadmin has no permissions.