I think you just go to "Home>Manage Account>", under "Hosting Plans", "edit" your hosting plan, then "close your account'.
Here's some advice I gave another poster on using DreamHost for pure domain registration:
So the problem is that after you close your hosting account, you'll need to find free DNS services for your domain. Until you find something better, use one of the ones I mention in my quote above.
To be specific and prescriptive, since you intend to close your hosting account, you should set up an account at one of the free DNS services I mention, after you do so, change your nameservers to the nameservers of the free DNS service, then close your DreamHost hosting account. You change your nameservers at "Domains>Domain Registrations>". Click on your domain listed there and "Modify WHOIS Info for yourdomain.tld" on the next page. Then fill out the section labeled "Set DNS info to host your domain elsewhere".
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