How can I stop the sync process from Salesforce to Campaign Monitor?

There are a few options:

1.  You can disconnect Campaign Monitor from Salesforce via our sync settings page
2.  You could disable all triggers via our general options page, then ensure no import wizard schedules are running.
3.  You could also turn off the sync using the turn off sync button on the sync settings page (this will simply stop the schedule - it's the same as setting it the schedule Never).

If the disconnect process does not work then you can also perform a connection reset in code.  Simply go to the developer console and execute this code which would perform a complete disconnect:

wbsendit.SendItAPI.resetSettings();