But sometimes it also happens that they just want solution X to be implemented

My response is usually that they should state in writing that they understand my objections and that they intend to override them anyway. Only with this in hand do I travel down the "you must" road. That way when it all goes tits up the evidence of the responsible party is in hand. The funny thing that i've found however is that managers dont like to do this for some reason and end up backing off. Its amazing to see the bravado vanish when you say "Fine. give it to me in writing and no problem." I suppose the thought that you might be right and they might be wrong is too much for them. But off course if you take this approach, make sure you are in fact right. :-)