To remove a syslog source you need to delete all the collected syslog messages (for that particular ) first.

1. Open the  Sources(x) tab:
   

   
   

2. Go to step 4 if the source has 0 messages in the Total column.
   
3. Delete all collected syslog messages for a particular syslog source
   
4. Select Remove Empty Sources:
   

   
   

CAUTION: This operation will remove all syslog sources with 0 messages in the Total column.