The Kitzur Shulchan Aruch (11:1) mentions that a door that is only opened on rare occasions (like when deliveries are being made) while most of the time another door is used for access to the room, then the first door is except from a Mezuzah.
It would follow that a door that you can no longer use should be except from a Mezuzah.
The following is based on the footnotes of the Sefer titled קביעת מזוזה כהילכתה Ch 8.
ifIf the door is simply sealed - and could be reopened - then the Poskim are in dispute if you can take down the Mezuzah. The Shevet Sofer YD(Yoreh Deah 92) and the Eshkol (Hilchot Mezuzah) allow one to remove the mezuzahMezuzah. The Igros Moshe YD I 177 requires it to have a Mezuzah unless it was permanently sealed with mortar or a permanent cupboard.
The footnote ponders what is considered sealed; what about if you lost the key, or placed a heavy cupboard in front of it or nailed it shut.
Some (Oruch Hashulchan 286:38) require the doorway to be destroyed - i.e. that the doorframe be removed, before it looses it'sits requirement to have a Mezuzah. Oruch Hashulchan 286:38.