How to force people to change their passwords?

[Lilmikeishere]

I want to make it so that every person has to change their password before being able to get on the forum again.

How can I go about doing this?

[notrious]

Annoy then by sending emails and pms.

[ChopSuey]

AdminCP>Usergroups>Usergroup Manger>Usergroup
First block you will see this

Password Expiry (If you specify a number of days here, users will be required to change their password when this amount of time elapsed since they last changed)

Password History (If you specify a number of days here, users will not be allowed to change their password to a value that they have used in the past x days.
Note: this setting has no effect if password expiry is set to 0 days)

Set to whatever you want.