but in all fairness, is a full day of work really too much to ask?
In a word: "yes" Personally, I have never, ever, contracted to give more than a full day's attendance for payment

OK, to the subject in hand:

If this is only one worker, or a few in a single office, I wouldn't bother to try anything too complicated, especially if I was expected to support and maintain it.

I would look at "parental control" software, particularly the "industrial strength" ones they use in schools. Sorry I cannot remember any names but at least one is very good at blocking all manner of subjects.

The beauty of the third party solution is that the vendors maintain it

Just a thought