Prohibited is easy. Just pass a law making it illegal for workers in that agency to strike, or to advocate, promote, discuss, etc., potential strikes (that may conflict with existing law, be politically difficult, etc., but the mechanism is clear.)
Prevented, though, is harder. Prohibitions aren't always effective prevention.
There are no agency workers who are participating in any strikes? Unlike the US we have a privatized postal service and the old postal service is now merged with the danish postal service and co-owned between the Swedish and Danish states and competing on an open market with other postal services.
Not everyone has state postal services like in the US
Prevented, though, is harder. Prohibitions aren't always effective prevention.