Scorpions and turantulas are in this house.
If they have phobias but keep coming, i assume they dont see your scorps or T's?
Anyways if your making a sign, dont put any red or bright colours on it, dont make the letters too big or bold, dont have a huge scary picture, and dont use the words "warning" or "caution"
Something like....
"Attention
For those with phobias or discomfort for arachnids, please be aware that the owner keeps both scorpions and turantulas on the premises."
and maybe
"for those who wish to avoid thes animals.. please use.... **insert way in etc**"