I had that problem. Ignoring didn't work, and I didn't want to block the person entirely because they did occasionally send a "real" e-mail. I finally sent them a polite e-mail explaining that I loved to hear from them. Unfortunately, I had very limited storage space in my e-mail account, so it would really help me if they would leave me off of the address list for the chain e-mails, jokes, etc. because the e-mails were using up my storage space and preventing me from receiving other e-mails. It worked.
Nowadays, I don't know if that excuse would work, as most e-mail accounts have huge amounts of storage (even the free ones). But you might use a slightly different excuse in a similar manner. Example:
"Dear highschoolbuddie,
I'm so glad that we were able to find each other using the alumni website. It's been great to hear from you and learn what you've been up to!
Unfortunately, I don't get a chance to check my e-mail very often, which means that the e-mails can pile up quite a bit before I get a chance to read them. Because it can take me a pretty long time to get through all of the e-mails, I've been asking my friends to exclude my address from any mass e-mails or chain letters. It would really help me to get through the rest of my e-mails more quickly.
So, did I tell you how my significant other was abducted by aliens the other day? Oh, it was such a hoot! [continue on with some blah blah blah story about your life to show that you're still interested in hearing from him/her, even if you don't want his/her junk e-mails]
Sincerely,
MamaMootz"