I have been volunteering with something that is organised through work. It is done during the working day and I commute to and from this activity using my own car. I would normally drive into work on the days when I'm not doing this activity so the fact that I'm in the car going to and from this activity would be the norm.
A colleague who is also volunteering at this activity has seen me as the taxi service to get to and from this activity.
I don't want to be seen as her chauffeur (or anyone elses either). I didn't mind taking her once or twice but every week that we're both doing this activity, she contacts me directly asking if she can get a lift.
So far today, from her I've had one phone call where she left a message, one missed call and one email all asking the same thing.
How do I get out of this situation while remaining friendly to her (as I don't want the atmosphere to change when we are both at this activity)?