Refusing to distribute mail to a specific recipient does not sound like a 'strike' in the sense of labour dispute to me. In France (often used a reference when it comes to striking...) that wouldn't be a legal strike, but obviously every jurisdiction is different.
In any case, this also probably breaches EU law related to universal postal service, which states that mail must be delivered to every address 5 days a week.
In any case, this also probably breaches EU law related to universal postal service, which states that mail must be delivered to every address 5 days a week.