Collect enough evidence to prove your point, then have a chat to them. They may be willing gradually to reduce their hours, or to work from home two or three days a week, or to shed some of their more onerous responsibilities - with an appropriate amendment to their terms and conditions. Or it may simply be that they have been doing what they are doing for so long, that they hadn't realised how slack they were getting. In most cases you should be able to resolve the situation to the satisfaction of both parties. If a reasonable and flexible approach does not work, however, you may have to make it a disciplinary matter. Take legal advice.
