To spurn means to reject or to snub or scorn or despise or to disdain. I imagine what the Lord means when he says "if ye shall spurn at his doings he will cause that it shall soon overtake you" (3 Nephi 29:4) is that we will become obsessed with spurning the Lord's doings that we will lose all reason and fail to see our shortcomings and eventually we will fall.
When the Israelites were bitten by fiery serpents, Moses raised the brazen serpent so that the people, if the looked on it, would be healed. But many spurned this counsel and consequently died.
We must be careful not to ridicule the Savior's teachings, else we become obsessed and begin to kick against the pricks and we are overcome with our own pride.
A Cry to Repentance
"Turn, all ye Gentiles, from your wicked ways; and repent of your evil doings, of your lyings and deceivings, and of your whoredoms, and of your secret abominations, and your idolatries, and of your murders, and your priestcrafts, and your envyings, and your strifes, and from all your wickedness and abominations, and come unto me, and be baptized in my name, that ye may receive a remission of your sins, and be filled with the Holy Ghost, that ye may be numbered with my people who are of the house of Israel." (3 Nephi 30:2)