How can I prevent a User from deleting the Tablerelation-Filter

So I am designing a Page where the User can select a customer and then a correlating Entry but he is also able to select an entry without a correlation if no customer is selected. I can easiliy do that by setting the TableRelation to this:



Now the thing is that the User can still clear the Filter on the Entry selection and is therefore able to select entrys that are not correlating to the chosen customer.


How can i prevent this? I know when setting the filter through Code I could use filtergroups but i hope to do it through the tablerelation