My spouse has aids. Can I refuse to have sex with him/her?

Yes, if your husband/wife has aids it is your right to refuse to have sex with him/her. In fact, it is your Shariah duty to refuse to have sex with him/her in order to protect your life. Allowing yourself to have sex with someone having aids is almost suicidal since you accepting to get infected and suffer a slow death. You are then also lowering or eliminating the chance of getting another spouse after death/divorce from the present one.  
Also, where it is proved that the other spouse has contracted aids due to illicit sexual activity, the husband can divorce his wife (or the wife can request for a divorce or an annulment of the marriage.