Recently we bought a FortiGate-200D VPN box. I have more good things than bad things to say about this device.
Long story short. I had to remove some users and because of some voodoo type of problem I couldn't do it from UI (I will contact their support that's for sure), so I had to do it from CLI. Who worked with Citrix Netscalers will find FortiGate's CLI a piece of sh!t (documentation makes no exception), but that's a different story.

• To display one or all users

fgw # config user local
fgw (local) # get | grep john.doe
fgw (local) # get john.doe
fgw (local) # get

• To delete a user

fgw # config user local
fgw (local) # delete john.doe

Note: When you're receiving an error like the one bellow the user is attached to one or more user groups.
The entry is used by other 1 entries
Command fail. Return code -23

In order to remove the user you have two options:

1. CLI:

fgw # config user group
fgw (group) # show
config user group
    edit "ssl-vpn_office_users"
        set member "user1" "user2" "john.doe" "user4" "user5"
    next
end
fgw (group) # edit "group_name"
fgw (group_name) # set member "user1" "user2" "user3" "user4"
fgw (group_name) # next 
fgw (group) # end

2. UI:
   You will have to login to the FortiGate webinterface, navigate to User & Device > User definition, edit john.doe and uncheck Add this user to groups