Everyone has the right to be represented by a Buyer's agent. The issue gets complicated when you want to get an agent after another agent already showed you the property. I have seen this many times...a lazy agent tells the potential buyer to go to see properties they might like and then call him or her if they like one of them. This really steams me because all the agent had to do is say, "hey Mr listing agent, I want to send my client to the property, but I am unable to attend myself". Problem solved.
Now things are not that cut and dry. Was the property in the MLS and if not, did the agent welcomed brokers in his ad? If the listing agent is not a member of LIBOR or whatever local organization then, he doesn't have to accept. Something else to look at, was the agent that showed you the property the actual listing agent? If the agent that showed you the listing is not the listing agent then he / she doesn't get paid unless he/she actually sells the property. In this case the agent that showed the property is 100% correct and he would be stupid to allow another agent to jump in on his deal.
There is a lot of questionable characters posing as professional and ethical. The truth is that there is a lot of desperate agents out there doing a lot of underhanded stuff to make ends meet. I have always welcomed agents, but If an agent comes to me saying he represents a potential buyer that already saw my property, I will give them a speech and then I will make him/ her earn his cut. I would never stop an offer to be presented, specially in this market...this would be unfair to the seller.