To prevent irrelevant keys from being offered, you have to explicitly specify this in every host entry in the ~/.ssh/config (on the client machine) file by adding IdentitiesOnly like so:
Host www.somehost.com
IdentityFile ~/.ssh/key_for_somehost_rsa
N. Alon, C. Avin, M. Koucky, G. Kozma, Z. Lotker, and M. Tuttle. Proceedings of the twentieth annual symposium on Parallelism in algorithms and architectures, page 119--128. New York, NY, USA, ACM, (2008)