Agree with above, also do you do Quality Inspections at regular intervals, check that they are doing the job properly, if they are leaving early and failing to achieve the required standard of clean, then you have yourself a discipliniary matter to deal with.
Do you have procedures, guidelines, expected standards of work that your staff have been trained on?
Do they understand that if they are paid for 2 hours work, then you and the client expect 2 hours work from them. If they fill in a timesheet saying they were there for 2 hours but in fact were only there for 1.75hrs, then they are defrauding the company and the client.
Whatever the results, it needs to be nipped in the bud before they start taking advantage of you and take it for granted that they can leave whenever they want.
In future, when you set up a contract, ensure that it is priced to achieve a desired standard and not based on hours on site, what one person achieves in a hour can be totally different to what another may achieve.
