help.yahoo.com —
Here's how you can add negative keywords to Yahoo Search Marketing campaigns that are setup to use the Advanced match type. Since "Standard" match type in Yahoo is essentially considered Exact match, you are not able to add negative keywords to a Standard match type campaigns ad groups.
"To use excluded/negative keywords, make sure that Sponsored Search is on and you've selected the Advanced match type for the ad group. You can set up the match type at the account, campaign, ad group and keyword levels. You can specify up to 250 excluded/negative keywords for an ad group. These combine with any excluded/negative keywords you specify at the Account level."
read more...