gajim has no option to disable the 'last seen' line option in MUC chatrooms
There is no option to disable the divisor line in MUC chatrooms that is meant to show how much of the chat has been read before.
Not everybody thinks it's a useful feature and it may be more distracting than useful. Also, the focus out mechanics of setting the line are not reasonable for some people who use viewports and focus follows mouse.
An advanced configuration option is fine for this.