If your rules change based on who broke them, people will stop following them entirely.

Selective enforcement turns a rule into a mood, and people adjust to a mood very quickly. Once everybody knows the standard depends on who you are, following it becomes a question of whether you will be caught rather than what is right. The rule you bent for a friend is the rule nobody obeys next month.

Look at the last few times you let something slide and ask who it was for. Apply the same standard to the next person regardless of how you feel about them. If a rule is too harsh to apply evenly, change the rule instead of the enforcement.