There are places the kohen may not be and things that he may not do, the torah places restrictions on the kohen that are not applicable to the rest of בני ישראל. These restrictions come in the form of negative mitzvot -מצוות לא תעשה- which are applicable in all times and places the kohen may be.
These prohibitions include two main themes;
All places and times
These mitzvot, along with all mitzvot that are an obligation of the body -מצוות שהן חובת הגוף- are explained by chazal as being applicable in all times and places.
Throughout the extensive texts of halachic responsa, rabbinic authorities take a stringent approach to these restrictions in the interest of guarding the kedusha designation of the kohen and going the extra mile to enforce his state of holiness when necessary.
The prohibitions of tumah to the dead and marriage restrictions are included under the מצוה theme of וקדשתו -“and you should sanctify”. While most instances of kehuna are considered privileges the restrictions part of it may at times be difficult to adhere to. So, can the kohen forego the topic and say “Ok, I don’t want neither the privilege nor the restriction”?
It is here that the strong arm of halacha is introduced, as is explained by the sifra;
ומנין שאם אינו רוצה דפנו? תלמוד לומר וקדשתו –בעל כרחו
And from where do we infer that if he doesn’t want (to be holy) force him? comes the verse to teach us “And you should make him holy” (even) against his will.
Another tone is found in tosefta, where from that same word of וקדשתו, we infer as to not allow the kohen gadol to degrade himself; רבי יהודה אומר אם רצה לנהוג בזיון בעצמו אין שומעין לו שנאמר “וקדשתו” -על כרחו, Rabbi Yehuda says; If he desires to act below par we don’t allow it, as it is said “vekidashto” -(even) forcefully.