Remove series=false from the options when loading biblatex and write this in the preamble:
\DeclareSourcemap{
\maps[datatype=bibtex]{
\map{
\step[fieldset=series, null]
}
}
}
This suppresses that field for all types of entries.
If you want to remove it only for a particular type of entries, let's say article, write
\DeclareSourcemap{
\maps[datatype=bibtex]{
\map{
\pertype{article}
\step[fieldset=series, null]
}
}
}
instead.
Remark 1. You have to switch to biber if you want to be able to use the code. Many biblatex features are unsupported if you use bibtex as a backend...
Remark 2. The option <field>=true/false is available only for the following fields: isbn, url, doi, eprint.
EDIT
As a workaround, when using bibtex as a backend, adding the following line to the preamble
\AtEveryBibitem{\clearfield{series}}
should be the right trick.