"If you have written the calls you are at the mercy of the purchaser. They can and will get called early."

But only very rarely, and only by complete option novices. Basically, someone who does this likes to give away free money.

Look at any random option in the money. Let's say the BRKB 405s expiring tomorrow. Someone (a novice) might choose to exercise the option right now and buy it for 405 with the shares trading at 406.10, they would get the shares for a net discount of 1.10. But why on earth would they do such a thing when instead they could simply sell the option for somewhere between $1.191 and $2.03 (current bid and ask). Heck, even if they want a quick fire sale, like instant execution, and they sell for $1.50, they would still extract more value from that option than by exercising it. But this is true for ALL options in the money, even with 1 day left, there is always a small bit of time premium in the option.