AFAIK the counter is really just a simple count of the number of entries in system view M_EXPENSIVE_STATEMENTS. I am not aware of any specific 'reset'-function for that.
However, for your production system it is not recommended to switch the expensive statements trace off.
See Martin Frauendorfer's SAP note
2180165 - FAQ: SAP HANA Expensive Statements Trace, Pnt. 8
Concerning the removal of entries - that's only possible indirectly as there is no option to DELETE FROM or TRUNCATE the view.
As laid out in
2088971 - How-to: Control the Amount of Records in SAP HANA Monitoring Views
the expensive statements view collects data into memory and also into trace files.
You can delete those trace files (expensive_statement...) and restart the instance to release the file handles and to clear the data collected to memory.