How do we get these spark plugs out?

My hubby has a 1991 Ford Probe and it started acting up a couple weeks ago so we started looking around at possible solutions to our problem and discovered that the spark plugs were totally saturated with oil. We started taking the spark plugs out and one of them is stuck and it will not come out. My husband tried for several hours and could barely get it to budge so he called his brother and an ex and they tried for about six hours to get the spark plug out. They tried soaking it with wd 40 to help loosen things up but it didn't help much. We are trying to sell this stupid car and really need to make this happen.