Sounds like your doing it correctly. What hostname are you using for SSH? if it's your domain name try SERVERNAME.dreamhost.com, if you are already using SERVERNAME.dreamhost.com then make sure your account is on that machine. (Where SERVERNAME is replaced with your actual servername)
Also remember the panel robot is delayed sending changes to the server. I haven't noticed it recently, but it used to often take longer than the 10-15 Minutes quoted in the panel in green text on the change successful page. Make sure you give a change from the panel plenty of time to become effective.
FWIW - if you change the User the domain is running from, this is true on shared hosting--not sure about other account types, its possible that your change of USER also caused the IP your domain is being served from to change. If that's the case try again after 4 or 8 hours for DNS propagation.
If all else fails type "what is my IP" in a google search box to get your local interent IP address. Open a ticket or chat with a support agent and make sure your IP isn't blocked for some reason.
Yes, you can delete all the users and start over completely. Keep in mind you won't be able to reuse the name.