Mailspring doesn’t currently have an IMAP folder subscription/unsubscription UI — all folders on the server are shown in the sidebar. The most practical workaround is to right-click on an account name in the sidebar and look for folder options, though hiding individual folders isn’t yet supported. This is a common request (similar to Thunderbird’s “Subscribe” dialog) and it’s on the list of things to add. For now, I’d suggest filing or upvoting a feature request if you’d like to track progress on this.