Galileo's advice is not good.

"Private land owners and companies acting on their behalf can’t clamp your car just because you’ve broken parking rules on their land. They can issue you with a parking ticket. They are also allowed to put barriers up to keep your car on their land or to stop you getting on their land in the first place." (from tinyurl.com/lh9ws9m - the Citizen's Advice Bureau)

A relatively cheap barrier that can be secured after the OP's colleagues leave would be better - maybe a chain and padlock strong enough - and tight enough - to cause problems. You could add a notice that cutting or breaking the chain would be criminal damage.

There was always going to be a downside to the abolishion of private clampers, not all parkers are sainted maiden aunts getting a prescription made up for a sick friend.

Peronally i'd drag the horrid thing out of the gate with a substantial vehicle, preferably leaving it on some double yellows or obstructing the public highway and deny all knowledge.

I doubt it.

I have done this on a couple of occasions. The owner has abandoned the vehicle so if left on private land I have had no compunction to drag it onto the public highway and then advise the local council that there is an untaxed car waiting for their ticket people. If the owner wishes to complain, then he can, but he will have to identify himself and open himself up to the monetary costs he was trying to avoid.

People who dump cars on other's land are effectively stealing.

I agree with Avant. A polite notice attached to the vehicle should be the first step. If that is ignored then it is definitely enemy action and other means employed. My wifes' employer installed the fold down metal post bollards which worked a treat. Good luck and let us know the outcome.

Concrete

Further to previous suggestion and if polite dialogue fails...

1)Measure exhaust outlet dia

2)Choose suitable sized potato and saw off two foot section of broom handle.

3)Ram said object well up said orafice using said tool.

I once watched a mate doing a three-point turn in his Mk2 Escort,after which his engine spluttered to a halt.He sat there turning the engine over until I approached him to point out that he had reversed into a grassy knoll and that he might like to take a look at his exhaust outlet.Cleared a sod of turf out and away he went. :)