I'm of the opinion that writing any protest phrase on the slip is a pointless exercise, no-one will read it, much less take any notice, much less care or bring it to the attention of someone who should care. The only effective protest vote IMHO is for everyone to spoil their vote or for everyone not to vote. That would send a loud and clear message to candidates that none of them evoke confidence in the electorate. Pigs will fly.
To me, a low vote only tells the politicians we don't give a damm about them and that they can go off and do what they like because we don't care. Instead what is needed is a very high turnout so they actually get the full picture of what the country is thinking