I dont want my clients to use Internet connection sharing.
Pls any clue to marking & dropping Natted packets.
Thanks in adv

The only thing you can do is set the TTL on packets going to their connection (their download) to 1. If it is a router on their connection it will decrease the TTL to 0 and discard the packet, if it is an end node it will process the packet normally. If they have a smart router configured right, they can undo that, though.
You can't detect NAT on normal connections, such as an HTTP download, by definition.

The only thing you can do is set the TTL on packets going to their connection (their download) to 1. If it is a router on their connection it will decrease the TTL to 0 and discard the packet, if it is an end node it will process the packet normally. If they have a smart router configured right, they can undo that, though.
You can't detect NAT on normal connections, such as an HTTP download, by definition.

Thankx Fewi
It worked.