Nazir 17a - L'av She'ein Bo Ma'aseh

The gemara assumes that if one would be in a cemetery and warned not to accept upon himself nezirus in a state of tum'ah and ignores the warning, he receives lashes. Tosafos questions why do we not consider this a l'av she'in bo ma'aseh since all he is doing is staying where he was, and the dibur of accepting the nezirus would certainly not qualify as a ma'aseh? Tosafos answers:

ולאו הכאי כלאו שאין בו מעשה במה שמקבל נזירות, דלאו שיש בו מעשה הוא במה שאינו יוצא משם כשנדור בנזיר

Tosafos understands that the decision to stay in the cemetery when he accepts upon himself nezirus is considered an action. Simply speaking it is hard to understand. The hagahos of R"b Ransborg explains Tosafos based on the Magid Mishna (Hil. Sechiros 13:2) that we categorize a l'av sheyesh bo ma'aseh by the type of l'av that it is, meaning a l'av that either can be or perhaps typically is violated with an action is called "yesh bo ma'aseh" even if now it is violated passively. Similarly the l'av of tu'mah which can be violated through an action is considered to have an action even when violated passively. The Gilyon Maharsha and Mitzpeh Eisan interpret Tosafos that we combine the original act of entering the cemetery (even though it was done b'heter) with the acceptance of the nezirus to qualify it as an action. This explains (as well as the Magid Mishna's approach) the halacha (y.d. 372) that requires a kohen who is in the house of a meis to leave immediately even if he is naked. We don't consider remaining in the house to be passive where there is a dispensation for kavod habriyos, rather we consider it to be active.

The nafak minah between the 2 approaches above is if a temei person who is not right now in a cemetery would be mekabel nezirus. According to the magid mishna since it is a l'av that is typically violated through an action, he would get malkus. But according to the other achronim that we combine his entering the cemetery, it is mistaver that it would only work so long as he is in the cemetery but if he has already left the cemetery just that he has a lingering state of tu'mah, the original entering the cemetery could not make this into a ma'aseh. This is very meduyak in Tosafos that the refusal to leave the cemetery is what turns it into a ma'aseh.