Calvinism relies heavily on the following statement:
Grace (specifically the grace provided via Christ's death) is sufficient for the salvation of all, but it is efficient for the salvation of some.
This is essentially all that stands between Calvinism and Universalism. The idea behind Calvinism is that Christ's grace is so powerful that if one is elected the pull of this grace cannot be ignored. And once applied this grace is too powerful to be removed. That's Calvinism in a nutshell.
So the question ultimately comes down to: if we believe that Grace is more powerful than sin than what do we do with people who are "Bought" but don't stay. There are two conclusions that a Calvinist may come to
These folks were not among the elect and were instead people attracted to the church for some other reason. Their conversion was not true.
These folks are among the elect but have strayed from the path for a time, but will in fact return in full grace.
Doing a bit of research the Calvinist consensus here is that the subject of this passage is the first. These folks claim to be be bought by Christ but are in fact lying, and prove it by their false teachings:
So to summarize the point here: false teachers are “saying” they are Christians, and “saying” they have been bought, but in fact are not bought at all. “Denying the sovereign Lord who bought them” is what they are saying about their lifestyle, though it is not true from the rest of the immediate context of the passage. The word “bought” means they are either saved (which we know false teachers are not saved so that cannot be the meaning of the passage) or they are “saying” they “they believe themselves to be saved” and ultimately are self-deceived. But it cannot mean that Christ bought them with His blood, and they reject that “offer” to eternal life.
This is far more consistent with the power, sufficiency and efficiency of grace than any other argument.