Even though Yahoo advises doing so,blocking emails by addresses is a temporary fix at best because as you have seen, each senders address is different. This happens because they are forged to begin with. This is part of what is known as email spam, which is the types of email thatyou are referring to recieving in your bulk folders. Subsequently the email addresses that you are blocking will be useless either because the spammer has altered the return address, or because they have been shutdown. You then end up with a large list of blocked addresses that becomes useless after a period of time.
Opening is not recommended either because if the email contains graphics/images, there are hidden tags embedded in emails that you won't see, but will send an alert back to the spammer that the email has been opened & that the address is "live/active" thus allowing the spammer to inundate you with even more spam.
Since they are being sent to your bulk folder in the first place, the easiest thing to do is to would be to leave them there & let them be automatically deleted. The default is 30 days, but you can change this using the instructions found here:Using SpamGuard Plus
Here are a couple of more links to read through if you have not already:Protecting Yourself From SpamBT YahooFAQsCommon Spammer Tricks & Tips to Avoid ThemBTW
: If I am not mistaken, you can see the full header of an email without opening it simply by right clicking the email & selecting view full headers