[rkangel]

Approach the problem not in terms of the hours they are working but in terms of the outcomes (whether that's not completing tasks, or lack of availability to clients, or something else).

For example, if they aren't delivering what they should: Have the conversation with them, explaining that they aren't completing the work they should be. They need to do something about this and one solution might be to spend more time it.

Note that this is also the early stage of what in the UK would be the normal process of dealing with under-performing employees, that might eventually result in termination.