The polling agent's job is to ensure that his candidate's interests are not jeopardized during voting - his job ends when polling station closes.
It's the counting agent who will witness the counting. The rules say an "X" against a candidate is a vote for that candidate but counting officers (who are civil servants) have been told to accept other markings such as "√" and "0". But these are subject to the mutual acceptance of counting agents. Mostly, the process is cordial because people know the odd unusual marking would not swing the results either way.
But sometimes, there are ballots described here, e.g. "F..k VN" next to the PAPzi team. Because there is general acceptance that anything marked against any candidates named may be considered a vote for that candidate, the PAPzi counting agent may try his luck to say that it is. Then it is up to the opp counting agent to challenge him, and would almost always be successful, i.e. it would be counted as a spoilt vote.
A friend who was ca for an opp team encountered exactly such a situation. When the PAPzi ca tried to claim the vote, my friend asked him, "you seriously want to count this curse as a vote?" The PAPzi ca backed down and that was a spoilt vote.
If you only fuck GMS and not PAP on the slip, it maybe counted as a vote for PAP by the polling agent.