Just put up a sign:
We serve ice cream...........
Once had a co-worker that walked into a kosher restaurant with an ice cream cone.............
“Once had a co-worker that walked into a kosher restaurant with an ice cream cone.............”
I guess it depends on the restaurant’s menu.
If meat-serving restaurant, I understand, as ice cream has milk (and mixing the two is a problem going back to the Caanites boiling babies alive in milk).
Or they simply don’t want food that they cannot prove is kosher on the premises, which I also understand.
Weird (and a bit rude) to walk into any restaurant with food from somewhere else, regardless.