ISPConfig 2: How can I specify which prefix new users get upon creation (e.g. web[no]_)?

Go to Management -> System Config -> Settings. On the tab ISP-Manager you will see the field User Prefix where you can specify a prefix for users. You can use one of the following placeholders:

[DOMAIN]

[HOST]

[WEBID]

If you don't want a prefix, just leave it blank.