Because soccer is a very inclusive game, youth soccer teams differ greatly in terms of ability and experience. Uneven match-ups will result on occasion (especially in tournaments), so you are likely to be involved in at least a few games each season where one team is far better than the other. At more advanced levels of the game, an occasional blowout will certainly be an unwelcome affront, but players generally have the maturity to learn a few lessons from the outcome and move on. When this happens in youth soccer, though, it is in the interest of all participants to level the competition in some way so that each player continues to experience a game that better matches the challenges of the game with each player's ability level.
To understand why this is the case, consider the premise that every game represents an opportunity for players to learn something. However, uneven matches may teach our players lessons we would prefer they avoid! For example, we want our players to approach each game with respect toward their opponent, and to never assume that a win is assured simply by "showing up". We also want our players to perform at their best in every practice and every game, so that we reinforce proper habits and work rate.
Unfortunately, when players discover they can give less than their best effort and still win, most will do just that. Conversely, when players perceive that even their best effort will have no positive bearing on the outcome of the match, they also tend to give half-hearted performances. Either way, every player involved in a match like this will have reinforced the wrong attitudes and habits required to develop as a player, and few will take any joy away from the experience.
How should teams and leagues deal with situations like these? One common approach, often called the "mercy rule" or "knock-out rule", dictates that a game will end if one team obtains a certain margin of victory (7 goals, 10 goals, etc.) at any point in the game. On paper, this policy appears to minimize the embarrassment suffered by the losing team, but the reality is that the players involved are effectively told "you aren't even worth playing for a full match"! Further, this rule does nothing to create a more appropriate playing environment during the minutes that were played, and it reduces playing time for all players (especially for substitutes, who may not play any minutes if the last few goals are scored in quick succession).
Another common suggestion is to simply tell your players to reduce their efforts at scoring more goals. Though this line of thought may be well-intentioned, instructions like "don't score any more" or "don't try so hard" send the wrong message and don't aid the development of any player. Telling your players to ignore obvious goal-scoring opportunities is arguably more disrespectful of the opponent than "running up the score", and will only lead to disillusioned players on both sides of the scoreline.
Instead, it is better to increase the difficulty for a dominant player or team to score additional goals by making a few modifications to the playing environment. If the win has been ensured, then the following adjustments can allow you to actually increase your demands on your players while also granting a more realistic challenge to the opposing team...
The key to success in these situations will always be found by looking at the problem from the perspective of player development. There is no single "right" answer to this problem, but applying guidelines like the ones above can help you turn a disappointing match-up into a valuable learning opportunity for everyone involved