How can I quiet my dryer vent from the outside?
Absolutely do not block the vent - this could cause a fire!

The first thing to check is that your duct is free of lint and other debris.  If it is a plastic duct then I wouldn't even bother cleaning it and instead would replace it with a metal one. A rigid duct line is probably better than a flexible one but is a bit more difficult to work with.

Check for any unnecessary bends or kinks in the line - the straighter the better as bends restrict the airflow and result in additional noise.  If it is a collapsible/flexible duct, make sure it is fully extended and trim off any excess ducting.

If possible you might consider relocating the exhaust further away from you neighbor.