We have serial upvoting and serial downvoting which are against the rules, but don't have serial unaccepting. Why?
The reason there is neither a mechanism nor an explicit rule against it should mainly be that it is rather rare.
One should not infer from this that it is fine to do it. Not everything is regulated in detail on an SE site but there are some general guidelines that always apply. Especially, Be nice. and Be honest. To unaccept something for no good reason is neither nice nor honest.
More specifically, actions focused on, especially against, one specific user are hardly ever okay. Generally speaking, all actions should be motivated by the content and situation at hand, not the user that created the content (1).
To unaccept answers provided by one particular user, only because this user wrote them, is thus not okay. As a special case of the fact that actions directed against one user are not okay.
Also, ill-moticated bulk unaccepting should not be okay and could when done in sufficient quantity count as vandalism.
That being said, for all I know there is no mechanism (not even a manual one) to reverse this, so there is not that much to be done. If you do not want to ignore it, you could signal the event to the moderators, via flagging one such post with "other," and explaining the situation.
They might then contact the user to explain the situation, and perhaps keep an eye on them. If it is severe they might treat it as vandalism and/or abusive behavior.
(1) I do not want to get into a debate whether in corner-cases with long time users this can be different. Please note the "generally speaking."