Hey Mark,

Some ideas for you for when (I won't say if) it happens again:

Make disgusting faces until they look away. If called on your rudeness, apologise and explain that your doctor has prescribed these therapeutic face exercises, and offer to demonstrate.

Attempt to engage them in friendly, normal conversation about the weather. If this doesn't work, start to drool mid-sentence.

Say "Keep staring. I might do a trick."

(Especially after the above.) Fake a seizure, sudden pain, or death rattle, followed by yelling "Ha! Fooled you!"

Ask them if they can spare some change.

all the best