Someone who purposefully kills conversations.
Someone who repeatedly accidentally kills conversations.
Someone who, by their mere presence, can ruin a conversation.
Someone who has trouble holding a conversation for more than a few minutes.
Essentially, a conversational grim reaper.
Can be shortened to "veeper," though that is even further from the original phrase.
"I was talking to my friends about this random mole that I saw in my backyard, and this douche John started talking about the mole on his arm. It's not even the same mole!! What a fucking conveeper. We just stopped talking and glared at him until he went away."
"I can never seem to keep my conversations going. I'm socially retarded! I'll be a conveeper for life."