If misleading messages ("phishing") are leading their users to enter credentials onto forms which are then used to send out spam, then the solution is not to block access to one of the sites that supports forms. There are an unlimited number of sites that support forms. There are LOTS of better ways to solve this problem. Here are a few:
* Train your users where it is and isn't safe to enter credentials.
* Don't give your users credentials. Have some alternate way to authenticate them like a login token.
* Put rate limiting on the ability of a single account to send out emails.
Blocking the site for just a few hours as an emergency response to a short-term attack is a much more reasonable approach. Sometimes, to react quickly, you need to take measures that are not the best possible choice. But there were better approaches, and the security team should take measures to ensure that they can react more effectively next time. For instance, in this case, a single mass-email or email "virus" had gone out and was tempting a large number of users to give out their credentials. Instead of blocking the site that was collecting the credentials, a better solution would have been to remove the email from the mailboxes of all the students. After all, the emails system is provided by the university, and this cuts off the problem at the root. They should institute the necessary technology to support doing this next time they have a phishing problem... perhaps they can even do this proactively: set up some honeypot accounts not receiving any legitimate emails and automatically destroy any emails matching the signature of emails received by these honeypot accounts (with manual review afterward to correct for false positives).