You can remove the desktop.
The desktop is in /slax/modules/
03-desktop.sb. If you rename the extension this will not load so there will not be a desktop. Therefor no auto-login as root.
You can load the modules but not start the desktop.
The desktop service is started, then /root/.xinitrc is called. This inturn calls startfluxbox. Remove or change
/root/.xinitrc to achieve the desired result.