I have to agree with some of the previous posters if the patient is alert and able to decide their own care then there is not much that you can do! May be a little more assertive be if the question is asked "Would you like to take a shower? 99% of the time the answer is going to be "NO!"
But above all DOCUMENT!, DOCUMENT!, DOCUMENT!, what you said and what the patient stated word for word, how many times you offered throughout the shift, if a bed bath or what alternative was offered, what reason the patient refused, and note that you did notify the charge nurse and have her sign next to your name for confirmation that she was made aware.
We have to remember as well if the patient is alert and able to make their own decisions there is that thin line that we are walking because it is a violation of the patients rights to refuse care!