OK, so you wish to block a IP range from your server using cPanel/WHM.

We've always recommended CSF for the firewall on your VPS running cPanel and to do this on CSF go to the ConfigServer Security&Firewall then do the following:

If the IP was: 12.345.678.90

Quick Deny Red block type in:

To deny IP Range: 12.345.678.xxx use: 12.345.678.0/16

To deny IP Range: 12.345.xxx.xxx use: 12.345.0.0/16

To deny IP Range: 12.xxx.xxx.xxx use: 12.0.0.0/16