If client want to use only squirrel or horde and he want to disable another then use following steps.

1)
If client want to disable horde.

root@server1[/]# vi /var/cpanel/users/username
add following lines

skiphorde=1
skipsqmail=0

save the file.

root@server1[/]# Service cpanel restart

If client want to disable squirrel mail and want to use horde add following line.

skiphorde=0
skipsqmail=1