zstyle ':completion:*:scp:*' tag-order '! users' 

This tells the completion system that, for scp, do not try to complete users unless no other completions can be found.

zstyle ':completion:*:scp:*' tag-order '! users' 

This tells the completion system that, for scp, do not try to complete users unless no other completions can be found. (See http://zsh.sourceforge.net/Doc/Release/Completion-System.html#index-tag_002dorder_002c-completion-style)

To specify a list of the only user names that should be completed when using ssh:

zstyle ':completion:*:ssh:*' users alice bob carol 

(See http://zsh.sourceforge.net/Doc/Release/Completion-System.html#index-users_002c-completion-style)