Hi Ankit, 

Great question - I am a BA Manager and currently also Head Of Change for a largish Financial Services company. This sort of thing regularly happens, so first off - don't worry.

Here are my steps for what I would do:

1. Those items on the backlog that are more than 1 year old - take them off and get rid of them. If, for each release, no one has declared them important enough to work on, they clearly aren't - so why keep them? If they are that important, someone in the business will raise it again. Really this is your POs job, but I get that he may be reluctant. However, he has to 'Own' the board. If something regularly doesn't make the cut there is a reason - its not that important. What makes you think you will ever have enough time for something so low in priority? You wont. 

2.Anything left, you and he need to prioritise. I assume you have already done this, but again, anything at the bottom, ask why it is you want to retain it, not why should you delete it. There is a difference in the two. If you cant think of a reason as to why keep it, then don't. 

3. You will know your burndown rate of your Dev team and your BA team, I would always keep no more than twice that if possible. Go through the exercise of point 1 and 2 regularly. By this score, also make sure that everything on that board has been allocated a priority score. It will make your life easier. Regularly review these scores

4. As with most things, starting this sliming down process is the hard part. Once you start, you'll be fine. You should both be telling the Business that they, the Business, equally need to own their work/requirements. If they have no wish to do so, then they probably don't care about it that much, so why keep it? Push comes to shove, go back and ask them if they still want xyz done? If they don't, scrap it. If they do, tell them that to add something to the board, they need to take something off. If they aren't prepared to do this, then it isn't that important. Remember though, they can only take something off the board that THEY added. They cant take something off the board that someone else put on there!

5. You say there are 300+ items on the board. That's way too many. Go through those left, and decide together (with your PO) if they are still relevant, or have they actually already been carried out under a different User Story. You'll probably find that a lot has changed since those were written, so you may well be able to get rid of quite a few. 

It sounds harsh, but owning a Project backlog requires both the BA and the PO to be strict, otherwise it can get out of control. Agree between you what the rules are moving forward, and stick to them. The rule about adding something only if you take something out is a good one as it will focus that persons mind to take responsibility for their own requirements.  

Remember, the PO OWNS the backlog. Not the business. So he (or she) has the right to say "Yes, that can come in", and "no, that can't". If the PO is worried that you might need something later on, what's to stop the business requesting it again? If you go through the steps I have mentioned above, you shouldn't have any scenarios of "we might need it in the future". If you don't need it now, you don't need it in the future. 

One other little tip. Often, the Business don't appreciate how many items are in the backlog. They only consider the ones they raised as being in it, so share the backlog with those stakeholders who have added something to it. This will again, focus their minds and you may get one or two saying that theirs can be dropped.  

Good luck, and feel free to ask anything else you wish to know.

Stewart