```python
from fastapi import APIRouter, Depends, HTTPException, status
from fastapi.security import OAuth2PasswordBearer
from typing import Optional

router = APIRouter()
oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/login")

@router.post("/logout", status_code=status.HTTP_200_OK)
async def logout(token: str = Depends(oauth2_scheme)):
    """
    Logout the user by revoking the access token.

    Args:
        token (str): The access token to be revoked.

    Returns:
        dict: A JSON response indicating successful logout.

    Raises:
        HTTPException: If the access token is invalid or expired.
    """
    # Implement token revocation logic here
    # For example, you can store the revoked tokens in a database or cache
    # and check if the provided token is revoked before allowing access to protected routes

    return {"message": "Logout successful"}
```

This code defines a POST endpoint `/logout` that requires an access token to be provided in the `Authorization` header. The `OAuth2PasswordBearer` class is used to handle the token validation.

When a client sends a POST request to `/logout` with a valid access token, the endpoint revokes the token (by implementing the token revocation logic) and returns a JSON response with a "Logout successful" message.

If an invalid or expired token is provided, the `OAuth2PasswordBearer` will raise an `HTTPException` with a 401 Unauthorized status code.

Note: You will need to implement the actual token revocation logic based on your application's requirements and authentication mechanism.