I share a narrow strip of right of way with my neighbour but it is never used. The strip of land runs around the back of my house and up my driveway, can I erect a fence between the two houses blocking this right of way?
Unfortunately no. If it's a right of way that is shared with your neighbour, even if they never use it, you still don't have the right to block entry to it. They could always change their mind about needing access.
Its a private right of way, to which your neighbour has a right of passage, presumably on foot. You would be committing a civil offence by blocking it (i.e. not a criminal offence). It may be possible to agree with them to put a gate in it to which they have a key, if you are both seeking to stop others getting round the back.