http://androidforums.com/thunderbolt-all-things-root/418539-thunderbolt-root-unroot-thread.html
since you likely have the old 1.04.2000 eng hboot from manually rooting,you can jump down to the "putting stock software and s-on back on the phone" and follow the directions from there.
all you need to do is flash the 2 files,listed,no hboot first,hboot only second.
since youre not sending the phone in for warranty or repair,id ask you to consider leaving the s-off hboot in place. it will not affect ability to recieve OTAs,or any other operation of the phone. it will also give the advantage of many more options to recover the phone if it ever becomes "soft bricked". plus if he ever decides to re-root,it will be easy.
flashing only the 2.11.605.9-no-hboot file will return to 100% stock,but with an eng bootloader.
holler if you hAVE any other questions