Renouncing Belgian Citizenship

To renounce your Belgian citizenship, you need to go to your commune and make a sworn statement that you no longer wish to be Belgian. It is not related to your taxes. Your tax status relates to where you are resident, not your nationality. You also need to have another nationality, as the state cannot leave you with no nationality.

If you have dual nationality you can simply use one and ignore the other; there is no need to renounce the one which you do not use.
You mention 'exit Tax returns'.
You liability to tax has nothing to do with your nationality, nor with your location. If you have a liability to pay tax here, it will still exist whether or not you live here and whether or not you have Belgian nationality. Similarly, any entitlement to a pension from the Belgian government when you reach retirement age will remain whatever your nationality and wherever you live.

I am wondering if you really mean citizenship or do you perhaps mean residency in that you are leaving Belgium. If this is the case then you have to deregister at the Commune where you live/are registered. I am sure they will also advise you about anything else you need to do but I would certainly contact the tax authorities to make sure you complete all the necesssary forms - especially if you think you might be due for any tax rebate. And of course contact all the utility companies - electricity/gas, water, telephones etc etc

In that case I guess you have Belgian nationality - in which case the other answers probably cover it.

The USA is the only country I know of that taxes its citizens even when they are not fiscally resident in the US.

Belgium doesn't do this. A Belgian citizen only pays tax in Belgium is they are resident here. If they leave Belgium and become fiscally resident in another country, they don't pay any Belgian tax.

So I can't see the advantage of renouncing Belgian citizenship. I'd hang on to it, as it should make travelling easier, and you may be glad of the rights it gives you later in life.

Agree with J. Can't see any tax or other issue that would require you or benefit you to give up Belgian nationality on a return to the US, so why do it? Unless you wish to get elected as President or to Congress and they have a silly rule like in Australia that dual citizens can not sit in Federal Parliament (however I do not think this is the case: Ted Cruz had Canadian nationality and Michelle Bachmann had Swiss - they both renounce for political opportunistic reasons, rather than any legal reason). A lot of people would love to have dual citizenship and need to fight and push to have Belgian citizenship granted. Can't see why someone would give it up after all the effort.