Hi Antoine
There is a global setting for seminars One message per date that should do the trick
Calendar support for .ics is non-standard and different email clients may handle these attachments differently. One known example is that MS Outlook handles attachments for seminar events with multiple sessions differently to Gmail. Therefore if your notification recipients use MS Outlook and you have seminars that have multiple sessions over different dates, you should enable the One message per date setting.
The disabling of iCal across the site isn't one of the Global settings - it is a user who chosses what type of calendar notification format they want when they make a booking on a seminar.
Regards