You can edit the file .spamassassin/user_prefs to customize SpamAssassin. Use the filemanager in your control panel to edit the file. You will not be able to see the file in FTP.

The file looks looks like this:

# SpamAssassin user preferences file. See 'man Mail::SpamAssassin::Conf' for
# details of what can be tweaked.
#*
#* Note: this file is never read by SpamAssassin. Instead, it will be copied
#* to a user's home directory, allowing them to perform personalised
#* customisation. If you want to make changes to the site-wide defaults,
#* create a file in /etc/spamassassin or /etc/mail/spamassassin instead.
###########################################################################

# How many hits before a mail is considered spam.
# required_hits 5

# Whitelist and blacklist addresses are now file-glob-style patterns, so
# "friend@somewhere.com", "*@isp.com", or "*.domain.net" will all work.
# whitelist_from someone@somewhere.com

# Add your own customised scores for some tests below. The default scores are
# read from the installed spamassassin rules files, but you can override them
# here. To see the list of tests and their default scores, go to
# http://spamassassin.org/tests.html .
#
# score SYMBOLIC_TEST_NAME n.nn

 

If you think too much email is getting blocked increase the number on this line and remove the #:
# required_hits 5

 

So it should now look like:
required_hits 10

 

If you think not enough mail is getting blocked lower the number.

required_hits 4

 

Just play with the number of hits until you have it to the level you want.

 

To see how many hits different items cause see:

 

For a complete reference of all the directives you can set see: