I assume there are laws against sending false distress signals. The protestors should be treated like someone who has just called in a false fire report, and charged with the expense of sending over the officers.
Inverted national flags are no longer commonly used as distress signals. It is difficult or impossible to determine when the flags of many countries (such as the United Kingdom or Argentina) are inverted, or the viewer may not be familiar with the flag and may not realise it is not upright. If a national flag is all that is available, distress may be indicated by tying a knot in it, making it into a wheft
http://en.wikipedia.org/wiki/Distress_signal