If you have been Stitched with another member but no longer want to see their messages in your Messages (inbox), you have two options.
You can Archive your conversation with them. This will remove the conversation from your Inbox, but the other member will still be able to send you messages in the future.
You can Block the other member. This will remove the conversation from your Inbox, and the other member will no longer be able to send you direct messages anywhere on Stitch.
To Archive a conversation or Block a member, click the menu on the right hand side of the conversation and choose the appropriate menu item, as shown in the image below: