A: A person who prevents you from seeing your clock or prevents you from getting somewhere on time.
B: A person who stands in front of the swim clock preventing another swimmer from meeting his or her interval.
Jack- "Sorry my roommate was being a clockblock and wouldn't let me leave till I cleaned the dishes from last night."
B: Coach- "Adam! You missed your interval!"
Adam- "Coach, it wasn't my fault. Mike was standing in front of the clock again.
Coach- "What a clockblock, I'll talk to him about it."