This is due to a newer security measure added in the couple latest versions of phpnuke, which does not allow html tags or several other codes to be used within blocks as it then creates more flaws in the block or the system.

FYI- Never use html in custom blocks as you are surely asking for a Hacker to rip through your site. If you would like more info in how to modify the main files to change these permissions please reply back with what you are looking to do as each tag will have a different code to change or add...

thanks