To disable the iPhone/iPad hotspot from starting when plugged into your Mac, open the System Preferences and click on Network. This will show a list of network interfaces your Mac has.
You will see something like this when the iPhone/iPad is plugged in (assuming you got a 3g/4g connection on it).
To disable, click on the iPhone/iPad and, then click the little gear icon at the bottom, then select Make Service Inactive.
You're not done yet, to make these changes permanent, click the Apply button.
The hotspot will not turn off on the iPhone/iPad and the connection to your Mac will be disabled permanently. The connection icon will be greyed out and will have 'Inactive' as a status.
To revert these changes, click the gear icon and select Make Service Active, then click Apply.