Disable StrictHostKeyChecking in ssh
To disable strict host checking on OS X for the current user, create or edit ~/.ssh/config and add the following lines:

Host [IP Address] 
   StrictHostKeyChecking no
   UserKnownHostsFile=/dev/null


A typical example for the hosts in your local network could be:

Host 10.1.1.*
   StrictHostKeyChecking no
   UserKnownHostsFile=/dev/null


Depending on your usage of ssh I don't recommend to disable strict host key checking for all hosts.

If you just want to remove the entry for 10.1.1.20 open ~/.ssh/known_hosts with an editor of your choice and remove the respective line "10.1.1.20 ssh-rsa public-key$"