Every organization has one, or several, or lots. People who just can't pull their weight. Maybe they don't have the skills, or maybe not the drive, or maybe they're misplaced. You can be certain that either: (a) they know they're not doing the job and are miserable each day or (b) they don't care. In the latter cae; who cares? In the former case, it's always a good idea to demote the person to a job he or she can do and then you'll have a great employee. But there's not always an available landing spot. So what do you do?
It seems harsh, it seems cruel, but letting go is the right idea. If you have a moral compass you worry: Will they find another job? How will they explain getting fired to friends, family, other recruiting places? Are there any jobs available? All valid concerns! Don't let those concerns cripple your decision making. Making and implementing a release is not assisted suicide; almost always it is a pass to a new level of self satisfaction for that individual. Receiving a clear message on their shortfalls is a relief. They'll land on their feet, and in a situation that fits their skills better. Help them out. Don't keep them in false-employment-prison. Let go!