[SalesForce] How to prevent password reset email from being sent when creating new user in API

If you are OK with having the initial email be the reset password then you can do the following which involves zero coding

Software

1. Data Load the Users
2. Go to Manage Users which lists out all of the users in the system.
3. Create a view that contains only those users in which you are interested.
4. Check all of those users' checkboxes (can also check the 'Action' checkbox in the header to select all).
5. Click the Reset Password(s) button.

On step #3 if you need to display more users you can click the More link at the bottom of the page and you can also hack the url parameter rowsperpage=210 to rowsperpage=500, for example.

After further investigation and running some debug logs, I found that our org had a managed trigger that was firing when the User record was being inserted.

We were not able to determine exactly what the trigger was doing (as it was managed code), but after disabling it the email is no longer being sent from Salesforce, and DMLOptions now appears to be working as expected.