Blood Bowl is a ferocious, no-hold's barred, bloodbath of a game even on the best of days. The rules were intentionally designed to encourage blocking and offensive play, and there is certainly nothing to suggest that Blood Bowl is a "gentleman's" game.
That said, there are certain styles of play that have found disfavor with the Blood Bowl community at large for being "unsporting" or "Unsportsmanlike." In the past, these have resulted in certain actions being banned from leagues or tournaments, and in players being marked down in the standings for "going too far" by attempting one of these actions. Without attempting to make a judgement about what is "Fair or Unfair" in a game of Blood Bowl, here are a few actions that have been called "Unsportsmanlike" in Blood Bowl as it is currently played. Players who wish to stay in the good graces of their fellow coaches might want to consider avoiding these actions, which might also be called "Beardy", "Cheesey", or "Twinky" plays. This list is constantly being updated. What that says about Blood Bowl as a game we leave to you to determine.
"Excessive Argument with the Commissioner"
Oftentimes, League Commissioners will be called in to resolve issues that arise during a League Match and are not easily resolved by the Blood Bowl rules as written. In all cases regarding rules decisions and a League Commissioner, the Blood Bowl rules are unambiguously clear: the Commissioner's word is law, period, and may not be debated.
Generally speaking, arguing a Commissioner's decision to an excessive degree is seen as unsporting and unfair. That said, coaches have been known to leave Leagues when they feel that a Commissioner has unfairly abused his powers.
Fouling is a normal part of the Blood Bowl game. It is used frequently as a tactic to incapacitate a threatening player, to even the odds on a pitch, or even to prevent a player who has been stunned from standing up again, thus clearing the way of potential tackle zones. There is nothing inherently forbidden about fouling in most Blood Bowl leagues, though it is worth noting that the consequences for being caught fouling in game for a player can be fairly severe (Ie: the ejecting of a member of the team from the rest of the match!)
However, there are actions which could be called "Excessive Fouling," over and above what is generally accepted as "sportsmanlike conduct." Typically, Excessive Fouling is defined as any act which results in a foul that does nothing to benefit your own team, and is merely done out of a desire to "remove more players from the pitch." This is actually a strategic blunder - no SPPs are awarded for fouling, and in any case, fouling with no good reason might result in the fouling player being ejected from a match, actually hurting the fouling side in the long run!
"Holding" is the strategy of holding the ball and choosing to avoid scoring, when scoring is highly likely (such as a situation where there are no opposing players near by, and the ball carrier has a clear shot to the end zone.) As a tactic, it is most often used as a means of maximizing SPPs by waiting until the last possible minute in a turn to run the ball in, allowing a team to skill up more effectively, or as a means of earning more benefit for the match. This in and of itself is not usually rated as a particularly unsportsmanlike act.
Excessive Holding, however is the act of holding the ball over multiple turns without running it in for a score. Because there is no "ball clock" or "shot clock" in Blood Bowl, a player could theoretically catch the ball in Turn 2 of a match, reach a position from which he might legitimately score, and then wait until the very last (or 8th) turn of the half to run in the ball. Doing so effectively removes the ball from play, and makes it extremely difficult for an opposing team to have any chance of scoring or effecting the outcome of the game. This is also sometimes referred to as "Ball Hogging."
"Timestopping" and "Analysis Paralysis"
This is the act of intentionally delaying the normal four minute limit of a Blood Bowl turn by excessively stopping the watch to discuss minor issues or to "double check the results of a roll in the book." The most accepted tactic to resolve the issue of a rule dispute or a minor quibble, especially when consensus or a Commissioner cannot be reached, is to simply roll a die, with the highest rolling coach getting the result that he or she desires.
However, some players have developed the tactic of Timestopping in order to extend a turn as long as possible in order to maximize their strategic options and to give themselves enough time to perform particularly complex maneuvers that wouldn't necessarily be possible within the constraits of a typical match. As the timed turn was made mandatory in the most recent versions of the game specifically as a means of preventing slow games and plodding players, many players now consider Timestopping, Analysis Paralysis, and even the refusal to use timed turns to be Unsportsmanlike in the extreme.