To deny His work OR to deny the work and person of Jesus Christ is the most basic description of blasphemy against the Holy Spirit.  For instance, the belief that "Jesus IS NOT God" is an example of blaspheming the Holy Spirit.  Basically, any statement or belief that challenges or rejects:

1. Theology Proper (the doctrine of God's existence and nature)
2. Christology (the doctrine of Christ)
3. Pneumatology (the doctrine of the Holy Spirit)

Many have mistakenly associated “taking the Lord’s name in vain” with “blasphemy of the Holy Spirit.”  The first is a forgivable sin while the latter ultimately is not forgivable.