Translation:They should not bring it
This 'rather strict assertion' mood (command or imperative) is dealt with in lesson 43 in KU course. But you can only give commands to people that you are talking to: ' go home now!', but not to third persons (he/she or they). So when using a- or wa- it is always the subjunctive, i.e. 'she should', 'they should'¶. And, of course, since you can't give commands to yourself ni- and tu- will translate to 'I should' and ' we should'.
The only time when there is ambiguity is when giving a negative command to someone that you're talking to: i.e. Usiipike or Msiipike which can be translated to either 'Don't cook it' or 'You should not cook it'. Usiipike is actually one of the exercises in this lesson, and DL accepts both translations.
¶ Plus a whole bunch of other possible translations, because the subjunctive is used much more commonly is Swahili than it is in English.
The object prefix i- can refer to a class 4 noun (e.g. "miti" = trees, i.e. plural only - so that would actually translate as "them" not "it") or a class 9 noun (e.g. "nyumba" = house, i.e. singular only).
It doesn't apply to the noun classes 1 (M/), 2 (/Wa), 3 (M/), or 10 (/N) that you mention above.
There is a complete table of subject and object prefixes here: