As Ethel points out, the first issue is 'Is there a statement in the Title Register of your property that grants each of the properties a right of access to their land through your land'. If there is, you have no legal right to stop him. Have a look at the wording and see what it says.
The second point is, assuming yes to the first, can you put a padlock on it. Yes you can for security reasons but you must give them a key. If they have a right of access, it will at any time and with no obligation to give you notice before they do it. It doesn't matter if they haven't regularly been using the access up unitl now - their rights do not disappear.