What do you mean?
I can think of at least two possibilities:
1) disable the 'pi' user from logging in. "man passwd" and look for the word "lock".
2) Prevent pi from automatically logging in on boot. You can disable the boot to desktop via the config program. However, regardless of how you have the "boot to desktop" functionality configured, the system may or may not automatically log in as pi on tty1 - depending on which exact release/update/version of Raspbian you are running.
3) ? There are probably others...
"L'enfer, c'est les autres"
G fytc hsqr rum umpbq rm qyw rm rfc kmbq md rfgq dmpsk:
J lnacjrw njbruh-carppnanm vxm rb mnuncrwp vh yxbcb!