Currently it's not possible to set the mainWindowHeight to values < 400. So if a users makes the window smaller than 400 px in height this change will not be respected.
reference: http://www.igniterealtime.org/community/message/156570#156570

One can anyhow edit layout.settings (layout.settings.xml seems to be no longer used) and set mainWindowWidth=210 to a small value to make sure that the Gateways will not be displayed (Y! will still be displayed). It may be nice to be able to hide the gateway bar completely even if gateways are installed with something like doNotShowGateways=1 for those who do not use them. A menu entry "Toolbars" would be even better.