General Nonsense:
* Non- (e.g., nonsensical, non sequitur)
* Pseudo- (e.g., pseudoscience, pseudophilosophy)
* Pseudo- (e.g., pseudo-intellectual)
* Anti- (e.g., anti-intellectual)
* Un- (e.g., unbelievable, unreasonable)
Specifically for words:
* Mal- (e.g., malarkey, malapropos)
More figurative options:
* Gibberish
* Balderdash
* Humbug
* Poppycock
The best prefix will depend on the specific context and the meaning you want to convey. For example, "nonsense" might be more accurately described as "unreasonable" or "nonsensical" in some cases.
Example:
* "The politician's speech was filled with nonsensical claims."
* "He presented a pseudo-scientific explanation for the phenomenon."
* "That's just malarkey, don't believe it!"