[SalesForce] Users required to reset their password even after System admin has set the password

Use a password management tool like 1Password, Lastpass, or KeePass. I believe all of them have desktop apps, browser plugins and mobile apps with database syncing in one form or another (such as DropBox).

I prefer 1Password as it's plugins make it very easy for me to use multiple logins for the same site. When I go to login.salesforce.com I hit a hotkey and it shows all my SalesForce accounts and if I begin typing it searches the account names, then I can arrow down and hit enter (or just hit enter if there's only one result) to log in.

It's the only sane way I've found to manage 50+ Salesforce accounts easily.

You could perform something like this in Execute Anonymous, silently changing their email after deactivating the user and then resetting the password:

// retrieve your 500 users
List<User> users = [SELECT Id, IsActive, Email FROM User WHERE FirstName = 'Derpy'];

// deactivate the users
for (User u : users) {
    u.IsActive = false;
}
update users;

// correct their email addresses
for (User u : users) {
    u.Email += '.nonsense';
}
update users;

// reactivate the users
for (User u : users) {
    u.IsActive = true;
}
update users;

// fire a password reset and send the reset email
for (User u : users) {
    System.resetPassword(u.id, true);
}

Prior to running this script, consider the repercussions of deactivating active users. SFDC Link: Tips on Deactivating Users