Two points to start:
Ideally question posts should be in final form when first posted. If everything goes according to plan no change at all needs to be made. Of course, some modifications and clarifications can be alright and desirable. Yet still the idea is that a question is posted in final form. (As opposed to the idea being that a question-post and an answer-post in some kind of symbiosis over several rounds of edits are grown to more complete forms.)
In particular, edits to a questions should not make existing answers invalid or incomplete.
As a rule "not so similar [...] question[s]" should never be in the same question post, not even when posted at the same time. The idea is more "one question per post" (not to be taken absolutely literally, but to get the idea across).
As a consequences of this what you describe is a malpractice for at least two distinct reasons. It should be avoided. Don't be shy to enforce it.
How you go about this is a matter of style. One way is to explain to OP that what they do may be a disservice to themselves. For example, like this:
I noticed you edited your post to include additional questions that are somewhat distinct from the original one. Posting them as a new post instead might give them more visibility. Moreover, users that have already provided answers here may be frustrated by their answers being rendered incomplete.
However, with users that are not susceptible to reason and do this repeatedly one might just rollback their edit to the original version.
As always there is some gray-area. What I wrote above applies to a situation of adding new and especially somewhat unrelated questions (well) after the original was asked and answered. If it is just a small complementary question motivated by an answer and no-one minds, it can be alright to add it. Yet even then, in case of conflict, the original post needs to stand and a new post should be asked.
