Let’s say, Alice and Bob have an open channel. Bob is a merchant. Alice is a client.

We create 2 LN nodes, lets call them Twin-of-death-1 and Twin-of-death-2.

Twin-of-death-1 creates a payment channel with Alice and only Alice. That means Alice can use this channel only to send money to the twin and no one else.

Twin-of-death-2 creates a payment channel with Bob and only Bob. That means Bob can use this channel only to send money to the twin and no one else.

Twin-of-death-1 makes a payment to Twin-of-death-2 routed through Alice-Bob channel with an amount such that the balance is 100% on Bob’s side now. Alice can’t use this channel anymore to buy things from Bob.

To rebalance the channel Alice would have to pay to herself in such a way that the final route jump is from Bob. She can’t rebalance the channel by routing through any of the twins because they are connected only to her. Maybe she can do that by another channel she has already open. Considering she has. And it is not attacked as well. And there is a cyclical route to herself through Bob.

If it’s not the case, the only thing Alice can do is to close the channel. Or maybe she can create a new channel just to rebalance the old one. Maybe this time, Alice, learned from the experience, will not allow any more nodes to connect to her — she would rather connect only to big trusted hubs.

TL;DR:

With a simple trick, you can force anyone who has less money than you to close their channels. A countermeasure for this is to centralize the network.