Deletion of Keys Page up and Page Down is not possible since they are part of the System Keys Definition. Hence, in order to disable the working of the same, we can change the action.
Below is the sample code for the same which changes the behavior of Page Down Key.
[#Form: Ledger]
Add : Option : MyLedgerForm : Not $$IsCmpOwner
[!Form: MyLedgerForm]
Add: Key : PgDn Display ReportNew
[Key: PgDn Display ReportNew]
Key: PgDn
Action: Field End
In the above code, the PgDn Key is altered to change the cursor to the end of the current field.