Usually the problem is not so much that such students cannot keep the beat, as they cannot hear the beat, that is they only hear the rhythm. (In other words they confuse beat with rhythm)
It is easy to check this out. Ask them to sing “happy birthday to you” and clap at the same time, and listen to see if they are clapping the rhythm or the beat.
If that turns out to be the case, then the best solution is the one that Appenato said above: lots of clapping (the beat of course). In the beginning they will not be able to do it, so you must do it with them and they imitate you (put a Cd with a strong beat and clap to it. Clap to the beat, and clap to the rhythm: make them aware of the difference). Eventually they get it.
If their perception of pulse is irregular, than you may have no other option but to use a metronome at the beginning (later on, once they develop a regular sense of pulse you can discard it).
That is the main reason I do everyday lessons. Otherwise (with this sort of student) you are fighting a loosing battle (the myth of Sisyphus comes to mind)
Best wishes,
Bernhard.