General antonyms:
* Unhelpful
* Harmful
* Detrimental
* Hurtful
* Injurious
* Counterproductive
* Ineffective
* Useless
More specific antonyms:
* Obstructive (if referring to hindering progress)
* Neglectful (if referring to ignoring needs)
* Uncooperative (if referring to refusing assistance)
* Malicious (if referring to intentionally causing harm)
* Unkind (if referring to a lack of compassion)
The best antonym will depend on the specific context in which you are using the word "helpful."