To remove your items from a Party, you have a couple of options. First, you can go to your Parties page. Click the Menu to the left of the Party name. Click Manage Items. 

Click the Remove Items tab. You can select the sizes or styles you would like to remove from the Party. Click Remove Items. The items will return to your Listings page. 

You can also remove your items from your Party through the Listings page. 

Select the items, scroll down and click Remove Selected from Parties. Select the Party name and confirm. 

This helpful tutorial video will also walk you through removing your items from a Sonlet Party.