How do I clear out the ssh-agent entries (on Mac OS X )?

I'm running Mac OS X, and it appears that after SSHing to several machines, using identity files, my 'ssh-agent' builds up a lot of identity / keys and then sometimes offers too many to a remote machine, causing them to kick me off before connecting:

Received disconnect from 10.12.10.16: 2: Too many authentication failures for cwd

It's pretty obvious what's happening, and this page talks about it in more detail:

SSH servers only allow you to attempt to authenticate a certain number of times. Each failed password attempt, each failed pubkey/identity that is offered, etc, take up one of these attempts. If you have a lot of SSH keys in your agent, you may find that an SSH server may kick you out before allowing you to attempt password authentication at all. If this is the case, there are a few different workarounds.

Rebooting clears the agent and then everything works OK again. I can also add this line to my .ssh/config file to force it to use password authentication:

PreferredAuthentications keyboard-interactive,password

Anyhow, I saw the note on the page I referenced talking about deleting keys from the agent, but I'm not sure if that applies on a Mac since they appear to be cleared after reboot anyhow.

Is there a simple way to clear out all keys in the 'ssh-agent' (the same thing that happens at reboot)?

Your SSH keys should not get automatically added to the agent just because you SSH'ed to a server...

Run ssh-add -l to list the agent's keys, ssh-add -D to clean out all keys.

Another way to limit what keys are checked against is to add IdentitiesOnly yes to /.ssh/config and a list of entries

IdentityFile ~/.ssh/<nameofkey>

You may also add them to each Host entry:

Host server-name HostName <ipaddress> User <username> IdentityFile ~/.ssh/<nameofkey>

SSH Man Page has more options as well.