To be able to make it impossible to register a service before a weaning you must go to “General”, then “Setup”, then “Validation” and then “Sow date”.  

Then you go to the “Farrowing to service (only when service before weaning)” and set both the red and yellow minimum boxes and the yellow maximum box to 0 or 1, depending on, whether you want to make it possible to register service on the day of the weaning or from the day after weaning, your window will now look like this:



Then click “Ok” and now it is impossible to register a service before weaning.