1. Britain can swap immigrants for retirees. The residency rights of all Britons in Europe are contingent on residency rights of Europeans in Britain. If Europeans are made to apply for visas, so will every one of the millions of Britons in Spain.
2. Drain the country of talent. Recognizing that millions of British are being unilaterally stripped of their EU citizenship, create a new category of general EU citizens, and allow talented British to apply for instant and permanent residence, conditional on (1) giving up British citizenship and (2) learning the language of their continental locale within the next five years.
3. Tell Scotland that it would accept their continuing as a member of the EU if and only if they applied, as an independent state, while the UK was a member state of the EU.
4. Tell the UK that Article 50 is considered to have been invoked as of now, and they have two years to leave.
The last two would be especially funny.