This is a common problem when you're seen as the go-to guy for something. If the constant requests for help or input are distracting you from your work it's reasonable to start enforcing boundaries as long as you clue your manager in first. When a pattern like this is established, your coworkers may be surprised or annoyed that you're suddenly pushing back and you want to make sure that your manager is both aware that you'll be doing this (as he may receive complaints/questions) and okay with it. It may be that this is a significant part of your job even if it didn't use to be and in that case you can't push back here. If that's the case though, it means that your "real work" is not as high of a priority as you thought and your manager thinks your time is better spent in a supporting role as knowledge or domain expert. If that's not what you signed up for it may be time to move on.
If your manager doesn't want you pushing back but still wants you to do all of your original work then that's a contradiction that needs to be resolved. The conversation there will boil down to "I have X hours available per week and if I dedicate Y to supporting my colleagues I'll only have Z left for my other projects. That means that A, B and C will be delayed." Your manager will have to decide the priorities.
Now, as for how to push back, I'll copy some excellent advice from Alison Green over at Ask a Manager. The full articles are definitely worth reading. From "How to say That's not my job":
There are times when it’s appropriate — and in fact necessary — to
communicate that you aren’t the right person to do something. That’s
especially true when dealing with coworkers, but it can be true with
you’re communicating with your manager as well (although usually that
should be rarer).
In doing that, you don’t want to simply say, “That’s not my job” — or
you would indeed risk coming across as being overly rigid. Instead,
you want to explain why you’re declining.
In your situation, I’d use language that refers to having other
priorities that you need to focus on. For instance:
“Right now I need to focus on X and Y so don’t think I can be of help.”
“I’m swamped and realistically don’t think I’ll have time to weigh in on this.”
“I’d have to spend some time digging into that to figure it out, and unfortunately I can’t right now because I’m on deadline.”
If you can, try pointing them in the right direction (like, “try
checking the X document on the server — it should help”). But if
that’s not feasible (because you don’t know or would need to invest
time in figuring it out), it’s fine to skip that.
More broadly, in some contexts you can try:
“I’m not usually the person who handles that. You might check with Jane to see if she can point you in the right direction.”
“I’m not usually the person who handles that. I’m not sure who is, actually!”
(Whether or not those last two are appropriate will depend on the
nature of your role. If you’re the CFO and someone is asking you about
making a change to the website, this is probably appropriate. If
you’re an assistant and your boss is asking you this, you probably
need to find out who the right person is to consult.)
The following is from "my coworker’s questions are getting out of hand" and while it's about a single coworker, the same strategy can be applied to multiple colleagues that have established a pattern of taking up too much of your time.
The easiest option is to stop making yourself so available to help her. When she IM’s you questions, you don’t need to answer them immediately. You can minimize the IM window and continue doing what you’re doing. Or you can respond back, “Sorry, right in the middle of something.” The same thing goes for questions in person — there’s no reason you can’t simply explain that you’re busy and can’t stop what you’re doing.
Alternately, you can address it big-picture rather than case-by-case. Say something to her like, “I tend to get really focused at work and having too many questions pulls me out of what I’m focusing on. I can answer the occasional question when it’s urgent, but could you direct most of your questions to ___ (fill in your manager’s name here)?”
In other words, while her behavior is absolutely too dependent on you, you’re contributing to the situation too. Be more conscious of your own contributions to it, set appropriate boundaries, and tell her directly when she should handle something on her own.
This one is also useful reading though it doesn't apply to this type of situation exactly: my coworker relies too much on my help.
As for the problems with notifying management about the situation: their reaction (department-wide mails about time tracking) is a classic example of non-management. You need to have a proper meeting about just this topic with all the people and managers involved to clearly define the priorities for you and your colleagues. They should have done this long ago when you first raised it but evidently it's up to you to spur them to action.