Hello,Tyrannosaurusss. It might be better if you posted that question to the "Learn Russian" section.Okay, I'll do my best in an attempt to explain.. Well, they are quite different units of sentence. In a one case it's a noun ("бессмыслица"/a nonsense). In an another it's adjective and needed an additional noun-word.("бессмысленный" поступок/nonsensical,meaningless deeds). And in the third it is a verb.("oбессмыслить"/make meaningless) The difference is pretty straightforward,huh? According that a common sense rule,you cannot use "бессмыслица" instead of "обессмыслить" in the straight interchangeable terms. Ex: a)"Это просто какая-то бессмыслица!" b)"Как тебе удаётся всё так обессмыслить?"

Dear, it is just different part of speech.