Are neighbours allowed to have their cement walkway up to the property line?

Technically speaking your neighbour is within their rights to build their walkway up to the property boundary (providing they are not violating any municipal zoning setback bylaws). However that right does not extend to them damaging any features or structures on your side of the property line. You have several pathways to resolution available to you:

1. Do nothing.
2. Resolve the situation with your neighbour.
3. Seek legal help. A lawyer who specializes in land and boundary issues can help you determine how to use legal means to assert your rights.