So uh, I want to erase Monika but I don't know how to do it and I'm on mac btw, so can someone help me please ?

you have to delete her files 

she say's it her self how she deleted the other girls 

if you are on mac you can right click then press show packet content - resources - autorun - characters- and then just delete her 

warning back the file up before deleting (recommended) so just click the ddlc file press characters then drag Monika's file to the bin

im stuck with monika too........i cant even find doki doki on my windows computer