Cubs to stand in two rows.  The rows face each other (one person opposite each other).  The one row will move closer to the Cub opposite them until the other Cub signals them to stop (when they start to feel uncomfortable).  They can then swop and do the same exercise in the opposite direction.

Discuss with them.

How did you feel when you told the other person to stop?

Why do you think some people were stopped far away and others closer?

Did anyone not stop when asked to?  How did that make you feel?

Discuss the importance of respecting boundaries (if you continue after you have been asked to stop e.g. teasing, then is can be considered bullying).  Also the importance of setting boundaries - saying no when something is uncomfortable for you.