JOEL -
"And in whatsoever place ye shall enter, and they receive you not in my name, ye shall leave a cursing instead of a blessing, by casting off the dust of your feet against them as a testimony, and cleansing your feet by the wayside." (D&C 24:15)

Jesus gave the same instructions to His disciples:
"And whosoever will not receive you, when ye go out of that city, shake off the very dust from your feet for a testimony against them." (Luke 9:5)

The act of shaking the dust off the feet as a testimony against those who reject the truth is an ordinance of cursing. It is not just a pronouncement that the truth has been given and rejected, it also means that those who rejected the gospel are on their own now, and that those who tried to teach them are no longer responsible for them.

"Behold, I sent you out to testify and warn the people, and it becometh every man who hath been warned to warn his neighbor.
Therefore, they are left without excuse, and their sins are upon their own heads." (D&C 88:81-82)

This is a rare ordinance historically performed by missionaries of the Church against those who rejected the gospel in a way that was offensive to the Lord, but done only when prompted by the Lord. Mission presidents today instruct their missionaries to not do this without first seeking permission from higher authorities.