That's unfortunate. Is it at least possible to remove the forum part of the account? Since if I want to post I'll do it in the Steam forum in future. And I hate keeping dead accounts alive.
Your customer/forum login is the same, as Island Dog mentioned. If one is 'deleted' both are.
The only option would be to ban your access to these forums which is more readily achieved simply by your not logging into them.
The advantage of a username remaining 'registered' is that no-one else can adopt your identity by coincidence [or by design] and thus cause confusion to others.
If you wish to remain off these forums either simply do so...or PM myself or Island Dog and your access can be removed for you...