As of now, there is no direct option to enable spamassassin for email accounts in windows plesk via web browser . Please follow below steps to do the same via command line:
1) Run cmd.exe as administrator.
2)Execute the following in the command prompt to use psa db:
Since Plesk 12.5, please use below command.
3) Run below query to output all the mail addresses to a file text.txt
4) Enable SpamAssassin using below command.
5) Enable antivirus using below command line