I very much doubt that any warning signs would deter most keen walkers, intent on reaching the summit of Snowdon.
Granted most of them do not see the danger before them, and those experienced enough are probably hellbent on reaching the summit come what may, and think that an accident will never happen to them.
Its impossible to lecture to people who do not know the associated risks of climbing such a high mountain in the grips of winter.
The wearing of costly crampons and the carrying of an axe, is something you do in the Alps, and after all, countless thousands of people reach the summit every year, so what is the danger.