Too much left-brain in most tries. The modern 'biohazard' symbol is not scary... nor is the ISO radiation sign. 'Skull and bones' is moreso.
Many old Hollywood films kept it simple with warning messages posted around territories ... heads on pikes, hanging skulls, that sort of thing. They say 'Enter here and you will likely die' pretty effectively.
Even George Pal's morlocks were scarier than that ISO sign. Looks like an ad for sunblock.