It sounds like you've given the user rights to that page, but possibly you haven't taken away the user's rights to the other pages.
The only way to do that is to put *all* the sites resources in a Resource Group and connect that Resource Group to a User Group the user is not a member of (e.g., Administrator), using a Resource Group Access ACL entry with a context of 'mgr'. You can use the DefaultResourceGroup extra to put future pages into that group automatically.
Be sure to flush both permissions and sessions after making any changes.
I recommend having only one member (you) in the Administrator group. It gives you more flexibility and helps prevent errors in assigning permissions.
If you have a lot of pages, this utility snippet should add all of them to that user group. Just put a tag for it on a page and this code in the snippet (create the resource group first):
$count = 0;
$groupName = 'yourGroupNameHere'; // Change to the name of your group
$docs = $modx->getCollection('modResource');
foreach ($docs as $doc) {
$count++;
$doc->joinGroup($groupName);
$doc->save(); // I don't think you need this, but it can't hurt.
}
return "Finished -- Added " . $count . ' resources to group: ' . $groupName;
I think you may also be able to use the Batcher extra to bulk assign resources to the group, but I'm not sure.