Its there a way to restrict zero entries in EWM?
Or is there a user exist in EWM like USEREXIT_SAVE_DOCUMENT in VL02N?
What is the business requirement here for you?
Do you want to stop creating zero items from S4 core itself? If yes- then this you can achieve by standard setup of delivery item category.
System won't allow you to create zero lines delivery from S4 CORE itself.
It's very normal that warehouse may need to change quantity to zero for few lines in EWM when items are not good to pick or ship. when you have mix of zero and non zero lines in EWM ODO or EWM OD and posts GI for the ODO or OD then system does create confirmation of service lines in S4 CORE for those zero lines once GI is posted from EWM.
If you are facing issues because of zero lines PGI then you can suggest users to delete those lines from EWM ODO for which they want to make zero quantities. This will be only helpful when you have situation to have a mix of zero and non zero items in your ODO (zero item you are going to delete). if you need to delete all items then you can't save the delivery.
Let's know what problem you are facing due to standard system behavior of PGI of zero items (Not to forget along with non zero items, because if you have all zero items only then there is no need to PGI, infect system won't allow to post PGI from EWM ODO or EWM OD if all items are zero line items).