Who are the "extreme Anabaptists" who believed that regeneration makes people incapable of sin? While reading Louis Berkhof's Systematic Theology, I noticed an interesting tidbit about the beliefs of "extreme Anabaptists" on regeneration.  He argues:


  [Regeneration is not] a complete or perfect change of the whole nature of man, or of any part of it, so that it is no more capable of sin, as was taught by the extreme Anabaptists and by some other fanatical sects. (4.6.C)


From this I gather that these "extreme Anabaptists" believed that at least some part of the "nature of man" was completely, perfectly transformed in regeneration, making the regenerate unable to sin.

This does seem outside the norm for Anabaptists, and it makes me wonder:


Who taught this, and when?
Did they consider regeneration to be something instantaneous (as in Reformed theology), or a process (like sanctification)?
Did they believe that the whole nature of man, or only a part of it (soul? spirit?) was transformed?
The belief you're interested in is Christian perfectionism. Perfectionism was held by a number of people during the Reformation era, and was especially widespread in the low countries. Advocates included Anabaptist Melchior Hoffman, the Reformed Sebastian Castellio, and the nondenominational Dirck Coornhert (Van Veen, 2004).

However, it's also worth considering the context for Calvinist claims of Anabaptist perfectionism. That name has been used as a derogatory epithet from the Reformation until the present.


  John Calvin's own frustrations with the Anabaptists
  had to do with the fact that the Anabaptists "out-Calvinisted" the
  Reformed community on at least two important points. The first was
  church discipline. The Calvinists were very critical of Catholics and
  Lutherans for their lack of attention to the role of discipline in the
  Christian community. The Anabaptists, however, took discipline even
  further than the Calvinists, insisting on very tight patterns of
  communal control. The Calvinists, obviously stung by this criticism,
  responded by labeling the Anabaptists as "perfectionist."
  
  -Mouw, R. J.
  (2000). Reflections on my encounter with the Anabaptist-Mennonite
  tradition. Mennonite Quarterly Review, 74(4), 571-577. summarizing Balke, 1981


As Mouw describes, Anabaptists can be understood as the extreme perfectionist end of a spectrum in terms of expectations for regeneration and personal transformation, so Berkhof may well be thinking of that spectrum without having any particular writer in mind.

While in this post I'm accepting that the label of perfectionism can be objectively applied to some writers, Perfectionism is usually a label applied by others while rejected by those to which it is applied (Augsburger, 1981).


  Often those Christians and groups of Christians who have honestly and
  earnestly sought to live a life of high dedication, obedience, and
  holiness have not only been misunderstood but also frivolously
  condemned as hypocrites or self-righteous. The attempt to strive
  toward perfection ("Be ye therefore perfect even as your heavenly
  Father is perfect") has often been erroneously labeled perfectionism.
  The Anabaptists and Mennonites have suffered under this charge from
  the beginning.
  
  -Harold S. Bender, Perfectionism, in Mennonite Encyclopedia.


To be thorough, you'll also want to look into early Christian debates about post-baptismal forgiveness of sins in early Christianity. Tertullian, was highly regarded among early Anabaptists, and they were surely influenced by his position.

For mainstream early Anabaptist authors perspectives on regeneration and quotes that could easily be interpreted as perfectionist, see especially Augsburger, 1981.

Further reading on early Anabaptist perfectionism


Augsburger, M. S. (1981). Concern for Holiness
in the Mennonite Tradition. The Asbury Journal, 36(4), 28-44. doi:10.7252/Journal.01.1981W.03
Balke, W. (1981). Calvin and the
Anabaptist radicals. Grand Rapids, MI.: Eerdmans.
Hughes, R. T. (1986). Christian Primitivism as Perfectionism: From
Anabaptists to Pentecostals. In S. M. Burgess (Ed.). Reaching Beyond: Chapters in the History
of Perfectionism. Peabody, MA: Hendrickson, p. 213-255.
Van Veen, M. G.
(2004). "No One Born of God Commits Sin": Coornhert's Perfectionism.
Dutch Review of Church History, 84, 338-357.