In him you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit, who is the guarantee of our inheritance until we acquire possession of it, to the praise of his glory. (Eph. 1:13-14)
And it is God who establishes us with you in Christ, and has anointed us, and who has also put his seal on us and given us his Spirit in our hearts as a guarantee. (2 Cor. 1:21-22)
The language here suggests to me that a "seal" in this sense should be thought of in executive/legal terms. Seals were used throughout history as a formal signature or badge which testified to a document or material's official authenticity. They were particularly used to finalize a law or issue orders. Any agreement or decision with the seal of a king or government is confirmed irrevocably (for example, Ahasuerus's seal in Esther). The only relevant sense in which this type of seal can be "broken" is when the physical document is opened by the recipient.
Therefore, from the above passages we should draw the conclusion that the Holy Spirit represents God's (The King of Kings) seal. He is the ultimate evidence of the Lord's signature/ownership of the marked individual. God specifically illustrates His divine authority by placing His Seal on believers. Moreover, Paul tells us that this Seal is to be understood as a guarantee of the inheritance - in other words, receiving the Holy Spirit is a promise to the individual himself.
God is not man, that he should lie, or a son of man, that he should change his mind. Has he said, and will he not do it? Or has he spoken, and will he not fulfill it? (Num. 23:19)
I can say with confidence that the promise of the Holy Spirit cannot be broken once the King has imprinted His insignia. In fact, I believe that to claim that His seal can be broken (read: invalidated or revoked) is to charge God with lying and failing to fulfill on a promise.