Swat!

A fly should be content outside
For if he ventures in,
He'll soon find out inside will be
The last place that he's been.

'Cause if he settles on the wall
Or window pane or sill,
His final glance will be the swatter
Ready for the kill.

A perfect world would not allow
Invasions by such pests,
But there's a reason you can purchase
Swatters for these "guests."