The pam_pwquality module's 'ocredit=' parameter controls requirements for usage of special (or "other") characters in a password. When set to a negative number, any password will be required to contain that many special characters. When set to a positive number, pam_pwquality will grant +1 additional length credit for each special character. Modify the 'ocredit' setting in '/etc/security/pwquality.conf' to require use of a special character in passwords.

[2_special_char/1_special_char/0_special_char]

Requiring a minimum number of special characters makes password guessing attacks more difficult by ensuring a larger search space.