In my understanding, vouloir asks for the subjunctive regardless of whether it is negated or not.

Vouloir que + subjunctive (Le Subjonctif) = To want someone to do something in French

Jane, I guess this is about the clause ‘.. si vous ne voulez pas qu’on parte/que nous partions tôt’. 

I am unaware of a rule that vouloir que in the negative does not use subjunctive. As you probably know,  there are very few expressions that use the subjunctive in the positive but not in the negative (douter que, for example) and a handful or so that use subjunctive in negative but not positive (penser que, trouver que etc).

Interested to hear from others but I would have used the subjunctive in this case.