Search engine spiders should only show up in the server logs not as a Guest user.
Anyway, as pointed out, why shouldn't guests be able to browse the forum?
If they just want to see what it's like before joining, or just cos they want to find a bit of information about a car, or even to find a vw to buy and thus join the VW community.
If they like what they see, they'll join so that they can be part of the banter and goings on.
I think locked down forums are very discouraging, and if guests aren't welcome, they won't stick around.