It seems that the only way to do it in 2.8.11 is by editing the plugins.
To edit a plugin, go to the LV2 directory (it can vary by installation but is normally /usr/lib/lv2) and open the directory of the plugin you want to disable the GUI for. In each dir there should be two .ttl files, one “manifest.ttl” and another one, usually with the same name as the plug.
Open the another one in a text editor and comment out the lines that call the GUI. Lines are commented out by adding a “#” at the beggining. In each plugin the lines that call (or define) the GUI are different, but they should be more or less obvious and they’re often at the top.