Unfortunatly removing the caffiene from coffee is not that simple, good decafeinated coffee would be a great deal easier to produce if it was. There are four main methods used for coffee decafeination.

Coffee beans are usually decaffeinated before they are roasted and are still green. The SWISS WATER® Process is one method that only uses water and no other solvents like dichloromethane. It takes 12 hours of soaking to remove the caffeine and the beans must be soaked in decaffeinated coffee bean extract during this time to prevent them losing all their other flavours. The caffeine is constantly removed from the liquid during the soaking so that more caffeine can be extracted from the beans. The beans are then dried and roasted.

If you try soaking coffee grounds you will remove the flavour as well as any caffeine. If you tried it with instant coffee granules they would just dissolve :)

THe main questions is probably if you are trying to just reduce the caffeine or you want to almost totall eliminate it. Might not be so hard to get some of the caffeine out and take the edge off, but getting all of it out would be harder.