However, the Returning Officer doesn't make it clear who scrawled "none of the above", and who put a cross in every box - so as protests go its entirely ineffective.
The ONLY people I saw questioning the validity of the ballot slips were the SNP - as I observed several slips which used their "x" symbol with a hoop at the bottom (see below)

instead of a plain "x". The SNP unsuccessfully argued that this should be taken as a clear vote - but in the counts I witnessed this was not allowed.
So, as I say, unless the candidates know what is "spoiled in protest" and what was merely completed by an idiot, then it is a waste of time going all the way there to do this.
You'd be better having a placard outside the polling place or the count centre - as then the candidates would know what your point of view is.