I am looking for Torah sources discussing protection from mazikim (negative spiritual forces/beings). So if, theoretically, one suspects a room or apartment has these things manifest causing trouble, how would one go about protecting himself or even better removing them from there besides checking of mezuzas and shema before sleep?
The Gemara in Megillah 3a; When discussing that someone feels frightened for no obvious reason, it is because their mazel (according to Rashi- a ministering angel) sees something to be afraid of.
The Gemara asks 'what is the fix? To recite krias shema.' In a place of filth that you can't distance yourself from, say 'there are fatter goats in the slaughter house than me.'
Halan yechidi belayla achazaso lilis, so as long as you have company in your flat you should be ok. Some people say leaving the light on in the hallway has the same effect.