Yeah, just restore to the 3.1.2 original firmware. Go here:
iPod, iPhone and iPad Firmware Download
Then click the drop down bowx where it says iPhone/iPod touch/iPad, find your device (1G, 2G, or 3G iPod Touch) and click the 3.1.2 firmware. This will then download the .ipsw file. Then launch iTunes, click your iPod Touch. Where it says Restore, hold down the Option key and click Restore. Then you can browse to where you saved the .ipsw file and it will restore it to 3.1.2 the original firmware (unjailbroken).
But you will lose all of your Media and Apps on the iPod, so make sure you backup.
I don't think you will get much help on these forums if you need help with a jailbroken device as it is against US law. I have a jailbroken iPhone 3GS so I can write some basic little Apps in C++ instead of using Objective-C. And I need to use a program called DragonFireSDK and you need a jailbroken iPhone to test your Apps. The AppStore has a few games developed in this and approved by Apple.
Anyways, don't talk about Jailbreaking itself here. And support developers by paying for Apps!!
Regards,
-Ben