¡Ni hablar! – No way! / Don’t even talk about it!
¡En absoluto! – Absolutely not! [This looks like you are saying “Absolutely” but in fact it is negative, you are completely disagreeing!]
¡En la vida! – Never in my life!
¡No me digas! – No way! [This can also express disbelief such as, “You must be kidding!”]
¡Qué va! – Not at all! / What rubbish! [This is a fairly friendly way of answering.]
Ni ahora ni nunca – Never ever [Literally: Neither now nor later]
Creo que no – I don’t think so
No es así – It’s not like that
¡Ni lo sueñes! – Not in your wildest dreams!
Literally: Don’t even dream about it! [Again, an informal way to say that it is never going to happen!]
¡Ni muerto! – Over my dead body!
¡De ninguna manera! – No way!
¡Ni se te ocurra! – Don’t even think about it! [This is informal expression and sort of telling someone off.]
!Ni en pintura! – Not at all. e.g. ¿Sabes cocinar? > ¡Ni en pintura!
¡Ni de coña! – No chance!
¡Ni de broma! – Not even joking!
¡Ni a balazos! – English equivalent – Not on your nelly!
¡Ni borracho! – No way! Literally: Not even drunk!
Hopefully one or a few of the above will help you to come up with different and more interesting and friendly ways to disagree or say no.