Hello Tajinder,
According to me the best way would be to trigger error pop-up through user-exit IEQM0003.
Code to be written in the include ZXEQMU02 , will be something like
If <your condition1> or<your condition2>.
message ' Different company codes at MaintPlant...' type 'E' display like 'I'.
endif.
With this whenever you create Equipment master the error pop-up will trigger and prevent from Saving , after the I0230 message appears. You need to give these conditions in the Condition1,2 etc (may be only one condition in this case) in the code. Hope you understood.
Though there may be ways through Tcodes such as SE91 etc to change the message attributes from Warning to Error (I do not know whether it is possible), it is not recommended.
Best of luck
KJogeswaraRao