As a Terran player, if I come across too much opposition when I start the harass I quickly abandon it and transition into longer play. As the harass is not a valid sustainable long game strategy.
So a couple spine crawlers is most likely enough to stop me at least. It all depends on how fast the first wave goes down. If it was effective enough, then I'll consider sending a second larger wave, if they die fast, its time to abandon that thought and try to salvage the econ situation into sustainable play.
Reapers are cheap, but slow to build. so they tie up the barracks for a while, and take a good chunk of time to amass into a large enough force to do considerable damage. They are also weak little sissies, so if you can get some damage on them they die fast.
If you've got enough static defense to stop the harass, a swift counter attack will be very effective. As the player has most likely spent the majority of their time producing reapers, however don't be surprised to see a few marines hanging out.
Also, if reapers exist, Marauders can too, so watch out for that.
Surprise can also be on your side if you catch them mid harass. As the reaper harass is intensive micro, if you want it to be successful. Just make sure you can deal with all of the reapers so that it doesn't become a base destruction race. You will lose that race, since you have no anti air and the Terran can just lift off their stuff.
I've been all over the place, but hopefully this is of some use to you.