Unable to turn off autopilot

I clicked on one of my freighters, and then, instead of clicking on a planet to send it to, I accidentally right clicked on one of my starbases. The freighter took off for the starbase, of course, but the problem is that I was unable to change the course until it arrived at the starbase. I tried clicking on it and then selecting a new destination 3 times while it was making its way to the starbase, but it would not be deterred from its original course.