Forum having 150 guests and being down (DDoS?) when this happends.

[Dontknowrandomnamefornow]

How can I stop this so my website can run properly?

Thanks in advance.

[Destroy666]

Is this a usual amount to your forum? If yes, you'll have to move to a more powerful hosting. If not, you can try ipbanning them.

[Josh H.]

In addition to what Destroy666 said, if they're abnormal and appear to be malicious, let your host know (if you're on shared hosting) and they should be able to facilitate IP banning at the Apache or server level for you. The latter would be more ideal because it would avoid Apache ever responding to a request from those IPs, but it could potentially ban IPs used legitimately on other sites.