Hi, you have the URL for your site? The solution you can have is to have a section where new members post before approval to become full fledged members to filter out spammers
To expand a little on haozwang's answer and give more detail for others who find this topic...
Create a sub-forum with a name such as "introduce Yourself" and in this area's description explain that all new users must post here before they get permission to post elsewhere. Point out that this is a manual process so may take time for a response. Pin this area and place it near the top of list of sub-forums. Also refer to the area in the description of the main forum page.
At the top level of your forum remove the "Create_topic" and "Reply" permissions from both "Anyone" and "Registered" and add the "Members" group. Now go to the "Introduce Yourself" area and remove those two permissions from the "Members" group and add them to the "Registered" group.
You will now need to add all those posting acceptable messages in the "Introduce Yourself" area to the "Members" group. It's worth subscribing to the "Introduce Yourself" area so you get instant alerts that let you know when spammers have posted so you can delete their posts and ban them and so you can add those who you want to accept to the "Members" group as quickly as possible. It's also worth sending a welcome email to those you accept to let them know they can now post in the forum.
Just a Volunteer Nabble Helper - because the nice folk at Nabble have helped me!