I have reset a member's password because I suspect they may not be authentic. The pop up box says an email will be sent to their email address notifying them that their password has been changed but the new password will not be provided in that email.

My question is: Would they be able to get the new password I just assigned to them by clicking on "forgot password" on the home page?

Unfortunately, the system did send out the new password to the user when you changed it, but as of now, it will no longer do that.

Some users can retrieve their passwords using the "forgot password" link. We are in transition to a system in which user passwords are not stored in plaintext and are not recoverable by the system. Users who have lost their passwords will need an admin to reset it for them until we implement a way to allow them to reset it themselves.

In the meantime, if you believe a user has hijacked an account on the system, you can remove the contents of the Primary Email address field (we recommend pasting it into the Admin Notes field so you have a record of it). With no email address, the user cannot log into the account. They will have to contact you to have their access restored.

OK, I deleted their email address from their "details" page. So, is it because they had once joined the website that they will not be able to just click on their name (on our classmate list) and begin registering all over again?

Right. Once the profile has been created by someone clicking their name and going through the wizard, that option is no longer available.