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:
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]