Try /ignore <nick> 3 Mirc will work out the adress for itself. If it still sets an ignore for 60 seconds I would take a look at any scripts you may have running. Oh and remember to change <nick> into the real nick of the person you want to ignore.
"/ignore nick" does set the ignore for an address for the nick without typing it out. However, the address it ignores may not be broad enough to prevent the same person from still contacting you. It's much better to ignore using the address format you want rather than using nick unless it's just for a very short-term ignore and you don't care if they get around the ignore.
