Make the home directory unaccessable by typing 
chmod ugo-rwx /home/dude



Make the home directory have the wrong name by typing
mv /home/dude /home/Dude   (i.e. use a capital letter)



Remove the home directory 
rm -rf /home/dude



Give them the wrong home directory
nano -w /etc/passwd
(Find them probably at the bottom.)
(Make the part that says /home/dude say /home/Dude (Use a CAP letter.))


Give the person the wrong password
passwd dude 
(Then pick anything stupid.)


Change the user id of the password entry
nano -w /etc/passwd
(Then find the person in the file probably at the bottom.)
(Then make the first number be 3 higher.)


Put an error in their /etc/shadow entry
nano -w /etc/shadow
(Then find them probably at the bottom.)
(Then remove a letter from their username.)


Mess up their group entry
nano -w /etc/group
(Then find them probably at the bottom.)
(Then change the name of the group to "YouBeHacked" or something funny.)


Mess up their group entry
nano -w /etc/group
(Then find them probably at the bottom.)
(Then change the number of the group to 12689 or something funny.)


Change the user's shell
nano -w /etc/passwd
(Then find the person in the file probably at the bottom.)
(Then make the /bin/bash be /bin/false.)