Staying more than 90 days.

Firstly, 90 days in 180 won’t start until 31/12 and freedom of movement ends for UK nationals.

But to answer your question, once Transition period ends and UK citizens become Third Country Nationals (TCNs) what stops you is that 90 in 180 refers to the entire Schengen Zone, not the individual country.  So even if you spent 88 days in Spain, then spent three days driving back to UK through France, you would be found in breach of the Schengen 90 in 180 for TCNs rule at the French border and likely fined and possibly banned from re-entry to the EU / Schengen area for up to 5 years.

Once transition period ends, there will most definitely be more attention paid to UK citizens time in Schengen Zone and if they have overstayed by any significant period.  1 or 2 days MIGHT not be an issue, dependent on the country you leave from - but a week or so will start causing issues.

When ETIAS comes online in 2023 that will be the death-knell for casual overstaying and getting away with it.

The 90 day Schengen Rule is really kicking in after transition period. Its 90 days in any EU Schengen Area Countries not 90 days in each individual EU country.

What is the point of it all? Is EU just being bloody minded about Brexit? 

“Bloody minded about Brexit” .... no, it is simply that UK is “leaving the club” and therefore no longer enjoys the benefits of membership, such as Freedom of Movement.

All that is happening is that UK is having applied exactly the same rules as every other Third Country Nation, i.e. America, South Africa, Australia, New Zealand etc etc.

'Third country'? 

Yes, “First country” is all EU member states, “second country” are those countries that are not EU members but are members of EEC, still enjoy freedom of movement etc e.g. Switzerland; everyone else is a “Third Country” - not in the club or affiliated to the club and do not enjoy any of the membership benefits

Ok with Schengen countries I understand but what’s to stop you flying back after 90 days from a Gibraltar ?

Very good point. Interested to see the answer.

There’s a border between Spain and Gibraltar.  Not much attention paid at the moment, you just wave your passport at the bored-looking police officers standing there as you drive through; that changes after 31/12.  You can expect long queues for all non cross-border workers as passports are checked ... I imagine Gib residents will have some kind of proof as well, so you can’t just pretend you’re a Gib resident returning home.  If you maintained an address in Gib, you MIGHT get away with it .... but have you seen the price of property in Gib?  Makes your eyes water.

Sorry ... no way around it.  After 31/12, if you overstay, you are probably going to be found out and if you are, expect heavy fines and possibly a ban on returning.