The Spanish equivalent would be "Porque si" (positive) and, as pointed out by rsanchez, "Porque no" (negative).
For example,
- Why did you do that?
- Because!
would become:
- ¿Por qué hiciste eso?
- Porque sí.
and:
would become:
- ¿Por qué no puedo ir?
- Porque no.
Now, there are other ways to express "because!", here are a few examples for its positive variation:
- Porque quiero (Because I want to)
- Porque se me antoja (Because I feel like it, a little rude - rude)
- Porque me da la gana (Because I want to, a little more rude - rude)
- Porque se me pega la gana (Because I want to, with even a more rude tone - rude++)
- Porque se me hincha la gana (Because I want to, with even a more rude tone - rude#)
- Porque yo lo digo (Because I say so, this is a favorite among parents)
Here are a few examples for its negative variation:
- Porque no quiero (Because I don't want to)
- Porque no se me antoja (Because I don't feel like it, a little rude - rude)
- Porque no me da la gana (Because I don't want to, a little more rude - rude)
- Porque no se me pega la gana (Because I don't want to, with even a more rude tone - rude++)
- Porque no se me hincha la gana (Because I don't want to, with even a more rude tone - rude#)
- Porque yo lo digo (Because I say so, this is a favorite among parents)
"Porque yo lo digo" works in both variations, that might be why it is a favorite among parents since it is so versatile.
Additional note: If you use one of the alternatives don't forget to conjugate to match the tense in which the question has been asked, particularly in the past.
For example,
- Why did you do that?
- Because!
would become (based on the conjugation of the verb "querer"):
- ¿Por qué hiciste eso?
- Porque quise (From "porque quiero")
However, in a future tense variant it is safe to use the present tense:
- Why are you going to do that?
- Because!
would become:
- ¿Por qué vas a hacer eso?
- Porque quiero (Not "porque querré")
Also note that "Porque yo lo digo" can apply as-is, without the need to match the tense.