[tor-relays] About running an Exit node

[Pika ohc]

Hi there,
   I was considering to run an Exit node with my own pc, however, i have some questions about exit node. 
1. How can I know if there is a client specifying me as an exit node and the traffic is sent from the client to me directly(where my exit node is the first node and also the last node for the client.)? 
2. If i found some clients trying to do something bad by using the method mentiond in 1., how can I stop him? Is iptables or anything else can help me to block such clients? 
Hope there's someone can answer me. Thank you! 		 	   		  
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