Thanks a lot everyone for your replies - I managed to solve the issue by using the code provided and all your suggestions for LEAVE to screen 0.
FORM user_command USING r_ucomm LIKE sy-ucomm
rs_selfield TYPE slis_selfield.
CASE g_ucomm.
WHEN 'DEMO' OR 'EMPL' OR 'ERROR'.
g_ucomm = r_ucomm.
LEAVE TO SCREEN 0.
WHEN OTHERS.
CLEAR g_ucomm.
ENDCASE.
ENDFORM.