What factors can stop new firms entering the market and competing away
a firm?s competitive advantage?

1) Patents
2) Control of components/raw materials
3) Contractual agreements with clients
4) Technical superiority - eg: massive research costs
5) Governmental intervention - eg: granted monopoly
6) Predatory pricing 
   - eg: offering low prices until a potential competitor is dead
7) Pulling strings at banks etc
8) Sabotage, arson, negative PR etc
9) Buy up the competition while tiddlers

This list is not exhaustive, however pretty recent examples can be
found all such techniques.

Personally, I am inclined to agree with Joe Schumpeter, that not all
monopolies are 'wrong' - or inefficient in the longer term.
'Free Competition' is an academic concept.