You put your own reputation on the line when you write a letter of recommendation. If you feel you do not know the person well enough, or they are somebody you cannot in good conscience recommend, decline their request.
The most effective letters of recommendation are written by professors or work supervisors who know you well enough to describe your academic, personal, or professional achievements and potential with candor, detail, and objectivity. Letters that compare you to your academic peers are often the most useful. Most schools do not consider general, unreservedly praiseworthy letters helpful.