Are we 100% certain Paul is referring to the Holy Spirit? One can logically come to that conclusion, but doctrine should be derived from multiple God Breathed scripture references. We can hold that belief, but are not certain.
I would challenge you to determine who else it can be. The verse sasy “He who now letteth”
2 Thessalonians 2:7 For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way.
Therefore it can’t be the body of Christ, the church and there is no other restrainer. The only restrainer of iniquity is the Holy Spirit and His work through the church.