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
E. Clouet. Acta Materialia, 54 (13):
3543 - 3552(2006)Selected Papers from the Meeting "Micromechanics and Microstructure Evolution: Modeling, Simulation and Experiments" held in Madrid/Spain, 11-16 September 2005.