removal of neighbor's farm sign on our property

We just purchased our 10 acre home on June 1, 2009. There was a "farm sign" posted on the corner of our property when we purchased our property. We thought it belonged to one of the previous owners, but found out it belongs to a farmer about 2 miles down the road. We were told we could remove the sign since it was on our property. We wanted to be neighborly and wanted to get settled in first before we asked the farmer if he would remove his sign.
On Father's Day, a gentleman approached the the turn off to the gravel road too fast and ran over the Farmer's sign. I should mention the sign was a 4'' pipe about 20' tall with about a 2' x 3' sign with the farmer's name on it. The sign and post was damaged beyond repair and we were thinking "good, the sign is gone, YEAH!"
We told the gentleman he needed to talk to the farmer, who is also his neighbor, to let him know he ran over his sign in our yard.
About 2 hours later, the farmer shows up and wanted to know if we knew what happened to his sign. My husband asked if the gentleman stopped by and he didn't. The farmer said he would be back to get his mangled mess. So, Monday morning, the farmer and the Amish are removing the mess. I waited to until they left to see if what sort of mess they left. The farmer left a 2 1/2' deep hole where the post had been cemented. I was was irked, but wanted to give the guy a chance to fill in the hole. Tuesday, nothing. Wednesday, still no hole filled in. I was worried that the Amish may pull their horses over and one would break it's leg and I would hate for that to happen. So, my husband and I filled in the hole Wednesday night, also placing some sod on the area so it wouldn't look bad.
The following Sunday morning , my husband comes home from work, all PO'd. He asked me if the farmer had been by. I said no he had not. He came early morning and put up a big shiney new farm sign without our permission. My husband was MAD! So he waited on the farmer to pass by the house and flagged him down. My husband asked when he put in the sign. My husband told him it would have been nice if he asked first. We would have told him "NO" because we didn't want the financial responsibilty if someone got injured or he was injured on our property. The farmer made a big fuss about his old sign was there over 20 years and he assumed he could put up a new one. But, if we wanted it moved, he would, but it would be so much trouble for him. WHAT?! My husband politely hinted it would be nice if he would. So, now we still have that sign on our property. He doesn't offer to come mow or trim, if we did let him, there would be a squatter's law, right?
I would like to know how to "kindly" go about removing this sign and what law can we show him to get it removed? As far as property insurance, who is responsible if some gets injured us or him??
We are in Johnson County, Missouri
Thank You for your advice!