Preventing selection of data during transaction

Hello List,

I wonder if it is possible to make data selected in one query NOT
accessible for a SELECT in another query?

The thing is that I have a cleaning script that selects some rows
from a table and processes them.
During that processing I don't want these rows to end up in another
result set. But I also don't want the other result set to wait until
my cleaning transaction is done, I just want it to ignore it.

Is this possible in the database level or do I need to take care of
it on the application level?

Regards,
Henke