First of all every time you Add a Join in a model you have to also perform the Remove a Join after your model no longer needs it before the first time you run the model. They should always go into models as a pair. However, since you already ran the model without doing that and then tried to rerun it in the same session the join has not been removed from memory. You may have to shutdown ArcMap to clear the join from memory and then validate the model again after reopening ArcMap to have it update. Then make sure you add the Remove Join tool before you run the model again. If any of the model inputs came directly from layers or tables stored in the ArcMap Table of Contents make sure those items did not store the join inside the map. If a join exists on those items, break the join in ArcMap.
