How do you evict the nanny?
If the nanny was living there, he or she could be considered a
"tenant" whose "rent" was deducted from salary. Therefore, you
would need to follow all local laws and regulations regarding
evictions (proper notice, opportunity to make other arrangements,
security for damages, etc).
If the nanny was not living there, then ask for the return of
any keys, give back any property owned by the nanny, and then
change the locks.