Q:

How do I prevent one user in particular from accessing my home directory? 

There's a specific user that I don't want to be able to access my home directory.  How do I deny access to them, while still allowing access to others who would normally have it? 

A:

If you want to limit access to users outside of your group or any other user, the question is a duplicate of Restrict access to my home folder from another standard user account
Otherwise, if you want to restrict access to just one user, call it enemyuser, use ACL: 
cd ~
setfacl -m u:enemyuser:000 . 

...and just enemyuser will have the access to your home directory denied.  
To check the effect: 
[romano@pern:~] % chacl -l . 
. [ u::rwx,u:enemyuser:---,g::r-x,m::r-x,o::r-x] 

To remove the ACL, you just say:
setfacl -x u:enemyuser . 

The *acl commands used here are from the package acl, which should be installed by default (at least, I think). 