Who actually enforces a post-employment shareholding requirement once someone has gone?

UK listed insurer. Our policy requires executive directors to hold shares worth 250% of salary for two years after leaving. It was approved three years ago and our first departure under it is coming up, a retirement at the end of the year.

The shares are partly in a nominee account we control and partly in the individual's own brokerage account from purchases made years ago. For the second part we have nothing but a signed undertaking.

How do others make this enforceable in practice? Do you require transfer into a nominee, and what did you do about shares already held personally?