I have no experience with the jury part of this, but the 2007 IOF Foot Orienteering Competition Rules state:
17.2
Out-of-bounds or dangerous areas, forbidden routes, line features that shall not be crossed, etc shall be marked on the map. If necessary, they shall also be marked on the ground. Competitors shall not enter, follow or cross such areas, routes or features.
17.3
Compulsory routes, crossing points and passages shall be marked clearly on the map and on the ground. Competitors shall follow the entire length of any marked section of their course.
Furthermore, the
overprinting symbols section of ISOM 2000 doesn't offer much more information.
The Sprint Map Standard
ISSOM 2007 (pdf) on page 22 and 23 has some "forbidden to cross" language in Symbols 707, 709, and 714.
It appears to me that there is a basis for argument if the route is not clearly marked both on the ground and on the map. I regularly see people miss finish funnels. Isn't that de facto proof that the funnel isn't marked well enough?
On the other hand, it does seem silly to require a mandatory marked route to have not only the marked-route-dashed-line, but also out-of-bounds or other "forbidden to cross" symbols if the organizers are serious about enforcing the marked route. Even then, if competitors miss them, they could argue the routes aren't well marked.