Can a bike wheel be damaged if it's hit by a mop? I can't keep my bike inside my flat, which is why I used to keep it in the staircase of my block.

The problem is, the janitor mops the staircase every day and I'm afraid he will hit the wheels of my bike with his mop.

I'm about to collect my bike from the repair service. They had to install a brand new wheel, because the old one was bent so badly it was almost unusable. While my bike is not new or expensive at all, I still don't want this to happen again.

Granted, my bike was subjected to an even worse mistreatment than just the janitor's mop, which was probably why the wheel got broken. But still, is it possible that this mop has contributed in benting the wheel?

Should I take measures to protect my bike from janitor's mop or may I ignore it?
I doubt the janitor could unintentionally damage the wheel with the mop by impact -- you don't use much force when mopping, and it would likely just turn the wheel or shift the bike a bit if it was hit. If someone drove their boots into the wheel or something, that could cause damage. 

One concern is the chemicals used in the mopping. Certain chemicals (e.g. ammonia) can attack rubber and cause it to degrade prematurely. Depending on the concentration and amount of use, this may be a non-issue.

However, if you are worried about your bike to this level, consider storing your bike elsewhere.