Probably, especially if you have a written employment contract and/or job description specifying that the position is full time. The other good news is that it sounds like the employee isn’t asking for the schedule change as an accommodation for a disability, parental, religious or other characteristic requiring accommodation under the human rights code.
The only potential risk I can think of is if you somehow represented to the employee that he/she might have the flexibility to work part-time after starting the job or there are any other indications of such a promise or understanding. Your last sentence about “the understanding they were hired for full time hours” is thus pretty significant. Can you document this “understanding” and prove that it ran in both directions? The more documentation you have, the safer you’ll be in terminating the employee for not performing one of the essential terms of the employment agreement. Hope that helps and good question. Glenn