Two points re your question:
1. Is the blocking action, although technically "illegal" by the book, creating an advantage/disadvantage?
2. Is the blocking action such that it could injure the player being blocked, (i.e., is it a safety issue)?
If you've got either, then maybe you've got a foul. Flagrant use of a forearm, for example, probably rates a personal foul as opposed to a hold/illegal use of hands foul.
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kentref
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