Yes, you can. First, navigate to your forum's Administration Area and click on the "General Settings" link under the "Customize Your Forum" section. Next, scroll down to the setting labeled "Disable Signups" and set the option to "Yes." To finish and save, scroll down and click "Save Settings".
Alternatively, you can use the "Limited Registrations" feature on your forum. When activated, it allows guests to create accounts which can only be used after you've manually approved them. You can enable this feature in the same "General Settings" area by setting "Limited Registration" to "Yes."