Struggling with ssh pubkey-auth on a 6248

Hi all,

I'm struggling to set-up ssh public keys on a 6248 switch. Unless I don't understand something correctly I was expecting to be able to configure the ssh server on the switch to make public key authentication a requirement.

Here's my config...

username "kevk" password d41d8cd98f00b204e9800998ecf8427e level 15 encrypted
crypto key pubkey-chain ssh
user-key "kevk" rsa
key-string row AAAA...
exit
exit
ip ssh server
ip ssh pubkey-auth

When I login with Putty and the key added to Pageant I get...

However I can still login as normal with just a password if Pageant isn't running?!

Thanks,

Kev.