The point of UBI though is to dismantle the administrative overhead and waste of existing welfare systems. A proper UBI implementation deliberately ensures it is administratively difficult to try and selectively remove people from the system.

You can't just replace a system where a very few people with real needs receive a large amount of targetted support with one where the same amount of money is distributed evenly throughout the entire population. Handing out free money is always popular; leaving people to suffer or die from lack of necessary medical treatment costing far more than the UBI payments, not so much. The introduction of a UBI would not automatically eliminate the need-based welfare system and its administrative overhead. Even if it did, in the end there would still need to be a system to keep track of who is qualified to receive UBI payments (even if it's just based on citizenship / residency), who has or has not been paid, where the payments are to be deposited, etc. Stopping one person's UBI in response to a court judgement would be trivial.