I asked the original question as I wanted to see what answers the community would come up with.
There are potentially hundreds of ways that you can humiliate your opponents in association football. Some other examples I can think of are
However, the key difference here is that all of these serve a purpose other than humiliating your opponents, ie. to maintain possession in a tactically superior position, or to score an otherwise uncertain goal.
When the ball is on the goal line, between the posts and all opponents are a number of metres away, the goal is practically certain. There is no other purpose in doing some otherwise difficult stunt than to humiliate and/or taunt your opponents, in order to "rub in" the goal.
On page 123 of the 2014/15 FIFA Laws of The Game:
There are different circumstances when a player must be cautioned for
unsporting behaviour, e.g. if a player:
...
- acts in a manner which shows a lack of respect for the game
...
While the following is in relation to the celebration of a goal, I believe that this point on page 124 of the 2014/15 FIFA Laws of The Game constitutes unsporting behaviour at any point in the match:
A player must be cautioned if:
- in the opinion of the referee, he makes gestures which are provocative,
derisory or inflammatory.
...
I find it hard to believe that any refereee would not find this to be "provocative, derisory or inflammatory" and acting "in a manner which shows a lack of respect for the game". It is mandatory for all football referees in the US to consider it to be unsporting behaviour, and generally considered by IFAB to be unsporting behaviour (here). On asktheref.com, which consists of a highly experienced panel of referees from around the globe, they almost all agree that is is unsporting behaviour (here and here) .
The correct action to take when such a stunt is performed is to disallow the goal, caution the player concerned for unsporting behaviour for acting in a manner which shows a lack of respect for the game, and award an indirect free kick to the opponents to be taken from any point within their goal area. Since the unsporting behaviour occurred before the goal was scored, cautioning the player and allowing the goal is not an option here, as on page 34 of the 2014/15 FIFA Laws of The Game it states:
A goal is scored when the whole of the ball passes over the goal line, between
the goalposts and under the crossbar, provided that no infringement of the
Laws of the Game has been committed previously by the team scoring the
goal.
To focus on this question generally, rather than the specific example given, yes, unsporting behaviour is in itself an offence. However, it is a cautionable offence, but not a free kick offence. Normally, a direct free kick offence is committed (such as a reckless tackle, or careless tackle for tactical purposes), and the match is stopped for that, and then the caution is given in the stoppage.
When a cautionable (or sending-off offence) is committed on its own and play needs to be stopped, the restart is an indirect free kick to the opponents.
An indirect free kick is also awarded to the opposing team if, in the opinion of
the referee, a player:
...
- commits any other offence, not previously mentioned in Law 12, for which
play is stopped to caution or send off a player
...
In a match in 2005 between the Netherlands and Andorra, Ruud van Nistelrooy taunted an opponent after scoring a goal. He was promptly cautioned for unsporting behaviour. The goal was allowed however, because the taunt happened after the goal had already been scored, not before. Arguably, the Andorran defender should have been cautioned for unsporting behaviour earlier, as he initially taunted van Nistelrooy after he missed a penalty. There is a video of the incident here.
As an aside, in your question, you mentioned offences that are "physical" or "verbal" in nature. While you are correct in that most physical offences are offences in themselves (usually resulting in a direct free kick), "verbal" offences are only offences if they are considered to be unsporting behaviour. There is no way in the Laws of The Game for a referee to punish any player for a verbal offences without cautioning the player for unsporting behaviour.
