dropbox – How to stop synchronizing a folder and delete it from the web simultaneously

My Dropbox folder contains folders containing temporary files that take up a lot of space. I can use "Selective Sync" to tell Dropbox not to sync them, but they stay on the web and use my space.
Is there a way to stop syncing a folder simultaneously and remove it from the web so that it does not fill my combo box? Or should I simply delete each folder manually using the web interface?