@Jacke: Looking at Tumblr, I can't see any way to block tags.
There are two main approaches to categorising posts. One is tags where a post can have multiple tags. The other is groups, where each post can only be posted to one group and the group has moderators who, along with the community, determine rules on what kinds of posts are allowed. Think of Reddit or Usenet.
They both have advantages and disadvantages. Tags are nicer for categorising and searching for posts, however groups allow a community to form around them. I suggest that one of the main reasons for Reddit's success was their use of sub-reddits (groups) rather than simply tags. StackExchange uses this model too, except with separate sites, not groups.