Trespass is defined as the unauthorised interference with the possession of your home and garden. The law says you have the right to decide what is left on your property, as well as the right to keep people out.
Most people protect their homes and gardens by building a fence, for which you won't normally need planning permission unless it is more than two metres high.
Signs that read "Trespassers will be prosecuted" are meaningless as you cannot be prosecuted for trespass - it is a civil wrong, not a criminal offence. It's therefore more accurate to have a sign that reads: "Keep out - private property. You are trespassing."
Reasonable force
If someone trespasses on your property despite due warning the practical remedy is to ask them to leave. If they don't you are entitled to use no more than reasonable force to eject the trespasser.
The right to use reasonable force has evolved through case law and there is no single answer as to what is reasonable - it will always depend on the prevailing circumstances.
However, you are not entitled to assault or harm a trespasser in any way. In the infamous Tony Martin case, for example, it was obviously unreasonable for him to shoot dead a teenager who had entered his remote farmhouse with the intention of committing theft.
In another case, a man who found a trespasser up a low ladder leaned against his house shook and overturned the ladder throwing the intruder to the ground. Although the trespasser was not injured, the judge said even this amount of force was unreasonable.
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Contrary to what most people think, homeowners actually owe a duty of care to trespassers to ensure they do not come to any harm, which means if there is a hazard on your property you might reasonably be expected to offer some protection. And you mustn't lay traps to put off trespassers or you could be liable for a claim for compensation.
http://www.theguardian.com/money/2007/jun/08/yourrights.legal