From what I know (but i might be wrong), archived data can`t be easily "recalculate"..so only your future visits are to be excluded, not past ones. There might be a hard way, deleting certain database tables and forcing piwik to rebuild archived data and MAYBE take into account the new rule you set, but i dont think its worth the risk to have ALL your data gone. Maybe somebody else can help you with this one.
On my site which is pretty old (8 monts) i had only two types of bots showing up: google and some seo bots from france. Excluded both via ip range and had never seen any bot in my stats in the past several months. But of course, user-agents is also a good way to get rid of official bots like yahoo, yandex, bing, amazon, etc without worrying of them changing IPs in the future.
Best of luck
On my site which is pretty old (8 monts) i had only two types of bots showing up: google and some seo bots from france. Excluded both via ip range and had never seen any bot in my stats in the past several months. But of course, user-agents is also a good way to get rid of official bots like yahoo, yandex, bing, amazon, etc without worrying of them changing IPs in the future.
Best of luck