Says who?
The validity of the election may be questioned if there is an unusually high proportion of spoilt votes, however, in countries such as the UK where a spoilt ballot paper counts towards the voter turnout, some voters will deliberately spoil their ballot paper to show disapproval of the candidates available whilst still taking part in the electoral process. In theory, a UK election could have a 100% turnout with no votes cast for any of the candidates if every registered voter were to spoil their paper, although this is highly improbable.
http://en.wikipedia.org/wiki/Spoilt_vote