There seems to be no rule against hitting an opponent and many players have included body shots in their repertoire. (This assertion is validated by the fact that players like the Bryan brothers and Ivan Lendl are patrons of this strategy and there appears to be no record of any punitive action against them for using it.)
If you are looking for more examples of players who used this strategy to win points, you can find a lot of examples in this 2013 article from Sports Illustrated.
"Where this really becomes an issue," [Brad] Gilbert said, "is in doubles. At close range like that, it's simply good strategy to hit the ball directly at someone. The Bryan twins [Bob and Mike] do that pretty often; you'd better be ready. Leander Paes [the accomplished doubles specialist from India] is a genius at it."
When Andy Murray enlisted Ivan Lendl as his coach two years ago, he quickly became aware of Lendl's penchant for targeting the opposition.
However, one risks losing the respect of fans and opponents on using this strategy often. In my opinion, opponents with good reflexes can sometimes return the shot thus making it a not so viable strategy in the long run, but the above examples show that it is a strategy that is used to some extent.