You can use the limitipconn apache module to limit the number of simultaneous downloads permitted from a single IP address

You can try the following steps to configure this module ( for Apache 2.X )

1) Download and untar the latest version of mod_limitipconn.tar.gz

2) compile and install the module using apxs as follows

/usr/local/apache/bin/apxs -cia mod_limitipconn.c

Add following line to /usr/local/apache/conf/httpd.conf , inside the virtual host of your domain

<IfModule mod_limitipconn.c>
<Location />

MaxConnPerIP 10

</Location>
</IfModule>

then restart apache

Hope this helps