You are able to manage your public portfolio via the Submissions section of your account (My profile > Submissions). Within this section you can:

1. Select a specific project from the drop-down list of projects you've participated in
2. Select a specific design and click on "Actions"
3. Choose to "Show" or "Hide" the individual submissions.

If the project you submitted to is a private project, your designs will not be shown in your portfolio regardless of the "Show / Hide" status you set. Designs only show up in your portfolio once the project has closed and all designs are finalized.

Note that a minimum of 20 designs must be shown in your design portfolio at all times (unless you've not met the 20 minimum yet), so in order to "hide" a design, you must at least have 21 designs displayed.