Latin sequitur is the third person singular present indicative of the deponent verb sequor 'to follow' (hence non sequitur means literally 'it does not follow'). Nomina agentis are usually formed on the the past passive particple form of the verb, e.g., amo 'to love' (actually literally 'I love') has a past passive particple of amatum 'loved', add the nomen agentis suffix and you get amator 'lover' (whence our amateur via French). Deponent verbs are verbs that are passive in construction, e.g., amatur 's/he is loved', but active in meaning, e.g., sequitur means 's/he follows', not 's/he is followed'. There is though a term secutor in Latin which means follower, so you could say something like nonsecutor, which in colloquial English has the unfortune to sound like non sequitur. Plus, that you can't really use non as a prefix. Or that the form should be more like 'a person who says non sequitur'. I think you'd need a word more like the Greek barbaros 'stammerer, barbarian, somebody who can't speak Greek properly'. There are some verbs in Latin that mean prattle, and you might form nomina agentis with them: e.g., crepo 'to creak, rattle; prattle' > crepitor (though the danger with this one is that it could be interpreted as 'farter'), adblatero 'to prattle, chatter' > adblaterator (cf. balteratus 'babbling, prating'), argutor 'to prattle' > argutator (though this has an established meaning of 'subtle disputant').
