Use of tick marks shows a lack of style and disrespect for other climbers who may want to try the same problem without them. They are an eyesore and, yes, they are a threat to access. Brush them off when you see them or after you use them. At the heart and soul of bouldering is the concept of rising to meet a challenge posed by nature. This is done by developing one's strength, technique, and mental skills.
Failure is part of the game.
Yet, when learned from, each failure is a step toward success. Remember, when out bouldering you are not in the gym. It's up to you to tweak your technique, not the holds. The boulders are your friends, treat them as such. Don't leave a mess.