When I log in to my modem webpage I can see lot of employees are connected to WiFi network. This is okay.
I have full access to the system and i want to disconnect the particular employee from the WIFI if he is downloading or using excessive amount of data. Currently i can see the IP address, MAC address, etc. of the connected users.
Is there any tools available or is there any other way that I can accomplish the task ?
It depends if your modem has a kick-off capability. (Probably not)
Just removing the IP from the DHCP list does nothing because the IP is already issued.
I think your best bet would be to block that IP in the NAT settings so it has no more internet access. Because even if you could invalidate his IP address his computer would just ask for another. Blocking it from internet will keep that from happening. He will still have local LAN access (to servers on your network) but he can't access the internet.
If your modem had QOS-capabilities maybe it's even better you limit his download-speed to a crawl. That way he can still access internet but downloading large files will just be very slow.