Ok there are many issues here. Firstly the EC cannot make a decision on an item unless it is listed on the Agenda of an EC meeting. The EC can discuss whatever they wish, but discussions are not decisions, and you cannot turn a discussion into a decision unless it is clearly listed as a motion on the Agenda of that meeting, a lot people tend to confuse the two.
An option would be to try calling an EGM; or if you have an AGM coming up, submitting a motion to be included on the agenda of that meeting. Any decision that is made in regards to the illegal works can be decided there, and any decision that is made at a General Meeting will overturn the EC decision not to pursue this matter.
Other than that, you could apply for mediation against the OC for not pursuing a breach of by law or mediation against that individual owner.