De acordo com a página man do Ubuntu para ~ / .ssh / config :
Host
......
A pattern entry may be negated by prefixing it with an exclamation mark ('!'). If a negated entry is matched, then the Host entry is ignored, regardless of whether any other patterns on the line match. Negated matches are therefore useful to provide exceptions for wildcard matches.
Assim, seu problema pode ser resolvido da seguinte forma:
Host !abc.com
ProxyCommand nc -X connect -x proxy.com:8080 %h %p