Occasionally, people have asked to be able to submit their ballots blank, as can be done in real-world elections, generally as a show of protest: https://en.wikipedia.org/wiki/Spoilt_vote#Intentional_spoiling.
For STV elections, SecurePoll currently requires you to rank at least one choice, and I assume this is true for support/oppose votes as well. Is this a design artifact, or an intentional choice? The downside I see is that allowing intentionally blank ballots would also make unintentionally blank ballots possible--we could mitigate this by having the software have it warn you and ask for confirmation if you try to submit a blank ballot, perhaps requiring you to fill a checkbox to make sure.