Ok, I will do thatn next time it happens to me.
It happened to me today in my race
http://igpmanager.com/play/?url=race-result/15911
I had a disconnection and when it reconnected, the "pitting" symbol was green (luckily I now know that it is a bug after a reconnection and it doesn't mean your driver will pit). ButI couldn't use KERS when I wanted even I had unchecked "Drivers controlled KERS".
I noticed that one of my drivers used it on its own even though I could not force him to do it.