How to lose a guy or girl instantly?

Tell that person that you want to marry someone 42 years your junior?

Say u have HIV/AIDS

Run, Forest, run!!!

Kick them out of your car and speed off.

Just say your gay or lesbian. But be serious about it.

Repeatedly mention how attractive their same sex sibling is, ask if they're single, do you know if they're a virgin? Have you ever caught them masterbating? What are your views on incest? Would you ever consider a... threesome?

Although that might take several hours/ days to properly execute. For a more direct approach tell them you once "experimented" with a pig, like good old David...

Tell them you have a kid.

Let them catch you fucking your or their mom-dad.

Disney world?

A hack saw, a plastic bucket and some acid always does the trick.