Yes, that is normal behaviour in PHPFusion. However, v7.01 will include a more advanced user management system, which will allow you to anonymize the user content if you want to delete the user. I cannot offer you a fixed date for the next release, but it will happen soon.
P.S. Right now you can just ban the users you don't want to post content.
Use 'ban' function included in current PF to ban the user(s).
Or, recognize the IP Address or the e-mail of the user(s), then record them into blacklist panel in admin panel. You should have the IPs in Members List section.
I don't know what you're trying to do. I thought you wanted to disable accounts without removing all the posts the users made in the past. That's how banning works. Now you want them to be able to post in the forums?
Please try to explain a little better. If English is the problem here, you could try asking the people on PHPFusion Russia.