Marko Ramius was the captain of a Soviet submarine in the film "The Hunt for Red October" who planned to defect to the United States. In order to ensure that there would be no going back for himself and his officers, he mailed the Soviet high command a letter explaining his intent the day before his launch. In the context of a relationship, to "Ramius" is to purposefully take an action or say something that makes returning to your previous relationship impossible. The most common instance of this is to leave your old relationship for a new one, publicly making this move known to your now ex-girlfriend.
Guy #1: My ex-girlfriend is still really into me, but we haven't been dating for a few weeks now. I am going on a date with a new girl tomorrow night. I don't know if I should tell my ex, because she might never talk to me again.
Guy #2: You should Ramius and tell your ex-girlfriend about it.
Guy #1: OK, I am going to pull a Ramius and tell my ex-girlfriend that I am going to date other people.