Its simple just research the L-Cluster Access Control technology pick the open option to keep the cluster open to and closed to close it. In 2.2 I will change this into a decision targeting the star instead If I can. But in order to change policy you must control Terminal Egress.

However if the AI has control over Terminal Egress by default they will keep the Cluster Open. This is to give you the player the chance to take control over Terminal Egress other players however well...good luck.

Also you should really maintain another gateway anywhere within the cluster so that you have some means of returning to the galaxy as if you keep the cluster closed and if you do not have access to other gateways yours ,your allies or netural gates then you truly will be stuck in the cluster until you open the Policy to open.

Now you may be wondering why add a benifit to keep it open? its to have an insentive for players to keep it open and to give the AI a little buff for doing so.