My son and his girlfriend are currently living in a static caravan on our property they are causing us no end of problems my son drinks a lot but we are wanting to sell our property so we have asked and now told them both they have to go to which all I get is verbal abuse they are flatly refusing to leave how can I get them removed

With the term "Static Caravan" being used, I assume you are in the UK. In the USA it would have been "Parked Camper". Location is important because laws and regulations vary.

Do you own the caravan or do they? As smoothy pointed out laws vary by area so any question on law needs to include your general locale.

But the generic answer is you give them written notice to vacate then go to court for a formal eviction order.

You treat them like any other unwanted dwellers on YOUR property. Local laws apply.

As has been noted, "Local laws apply". But in general, there would be two ways to go:
1. Get the police, sheriff, constabulary, or whatever to arrest them for trespassing; or
2. Get some sort of a court order and then get the police (or etc.) to enforce it.