Here's how to get them out of the way:
1. Talk to the person: The person is a young boy who will tell you about a Diglett who is blocking the entrance to the cave. He'll then ask you to catch the Diglett.
2. Enter the cave: Go into the cave and you'll find the Diglett blocking the path.
3. Catch the Diglett: This Diglett is level 5 and can be quite a challenge at this point in the game. Use your best Pokémon and try to catch it with a Poké Ball.
4. Return to the boy: Once you have caught the Diglett, return to the boy outside the cave. He will be grateful and move out of the way, allowing you to enter Diglett's Cave.
Note: You do not need to keep the Diglett you caught. You can release it if you want.