And have no assets in America, or do any work for or with any American company.
If you lose a case in an American court they can from memory basically go after any assets you have that are held for you by anyone or any company that is based in (or possibly just has offices in) the US, in addition to any potential attempts to recoup the money via international agreements/foreign courts.
So he likely wouldn't be able to do any work for any american companies (IIRC he has various business ventures with American partners), or have any money in American banks without it being at risk of being seized to pay if he loses the case.
He's not some numpty who has skipped the country with a debt of a few hundred dollars, if he loses the case there is going to more than enough incentive (especially as the costs of recovering any compensation would likely be added to the amount) to make it extremely likely that the lawyers will use every avenue to collect.
One of the stupid things is that his utterly ridiculous attempts to avoid even receiving the paperwork has made it if anything far more high profile than it would have been, and that interview he did was so bad that I saw lawyers who were pretty much saying "does he even have legal advisors?".
He's always been a bit of an embarrassment to the Royals* (far worse than Harry ever was), and he's just making it worse.
*Spitting image in the 80's knew of and mocked his reputation with a song from memory.