Do You Block Spiders?

Do you block certain spiders?
If so, why?
I have been getting hits like:
Hostname: spider-199-21-99-199.yandex.com
Hostname: ec2-52-4-176-40.compute-1.amazonaws.com
Hostname: static.227.10.9.176.clients.your-server.de
Montego Bay, Jamaica (multiple hits)
Hostname: 207.204.122.220
(France)
Hostname: 195-154-240-246.rev.poneytelecom.eu
(more foreign, then US, and […]

See original post by invalid@example.com (dscurlock)