There is no filter on Yahoo Answers, sorry. You can customize your feed (the gear symbol at the top right when you're looking at the category list of questions) but it doesn't include a filter. This is not the first spam-bot attack YA has endured, but it's the worst because when the previous ones occurred, there was staff behind the scenes to take care of the posts before they took over the site. Alas, this time the powers-that-be appear to be unaware or uncaring. We've certainly no indication anyone is paying attention or intending to take action, but that doesn't mean it won't happen.
This YA site is for English language posts only, but spam bots posting in (what we assume is) Chinese have flooded the site. There is no way to avoid seeing their posts unless you happen to frequent a category that hasn't been hit or has been cleared of those posts due to reporting or moving the posts to Travel>Asia Pacific>China.