Daniel Christopher:
mdlElement_undoableDelete
Daniel Christopher:The name contains a clue: if you delete an element using that function it will be undoable.
MicroStation's editing tools all fill the Edit|Undo and Edit|Redo command buffers. It's what a user expects. Why do you want to change that idiom?
MicroStation Compress Command
Take a look at the Compress command in MicroStation help: Compressing a DGN file reduces its size and clears the undo buffer.
The MDL equivalent is mdlSystem_compressDgnFile. Proceed with caution.