Go into your control panel and call up Forum Settings. For each forum, click details. Under Who Can Post threads and Who Can Post replies, choose the dropdpwn choice for any users (including unregistered). Then any user, reg or no, can post using anonymous or any other unregistered name.
You can turn off registrations completely, but I do not recommend it as it complicates administration and may lock out registered users. You can also suspend registrations. That will allow current registered users to post normally.
Spend some time browsing through the General Settings so you can see what options are available.
