What does 泉 mean in 道具泉, in the context of a handbill?
Obviously it was referring to the host 關灝泉 Kwan Ho Chuen（香港中文大學哲學系博士生）

In Hong Kong, people often nickname someone by his job. For example. 李先生 is a 經紀, people who mainly know him as a 經紀 might nickname him 經紀李 

More examples: 

張家強 is a 豆腐店店主，people who order tofu from him might nickname him 豆腐強

黃國興 is a 律師， people might nickname him 律師黃

In the video the guy asked 關灝泉 to hand him the placard, jokingly called him 道具泉 because he was responsible for the prop (jokingly referred him as 名字中有個泉字的道具部員工- An employee of the props department with a word 泉 in his name) 

It is a casual way to nickname people. For someone with high status, it is considered disrespectful to nickname him this way. You should never call 黃律師 as 律師黃 in his present.