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You can open a gate to prevent access to your property if you wish, but you really shouldn't have to do this.
What you could do, however, is write to the neighbour concerned and make clear that if he continues to trespass onto your property in this way, then you will take court action if you have to.
Then if you wanted to, you could seek an injunction against him in the court, which means that if he keeps trespassing once the order has been made, he would be in contempt and could be punished by fine and/or imprisonment.
Yes, do a simple letter than threatens to get an injunction if he doesn't stop. The costs depend on the lawyers you use, but on average, for something like this which is no fact heavy or difficult, I would have thought something like £3,000 to £5,000 plus VAT would probably be about right. If you succeed, you should get an order that your costs be paid by him too.