By continually submitting conflicting transactions. Network latency means its very possible that I'll get two locker signatures for the same spend in consecutive rounds. Then I provide the proof, and drain their accounts.
The lockers are always up to date with the state of the accounts they oversee. When you submit two conflicting transactions, only the first one will be signed by the lockers (the second one will be rejected). Please read the paper again.
How is that possible? With 10000 transactions per second, you can never guarantee consistency like that. 'First' is relative when you're dealing with latency.
According to the paper, lockers are chosen at random anyway, so you've got the potential for different lockers to be signing the same spend in different rounds.