I think a good (and easy) method would be to see if any of the tags they are trying to input can be matched at the beginning or end of any of the others, and then disallow the longer version (or suggest an alternative).
Using your example above, photowalk matches the beginning of "photowalk2009" and the end of "worldwidephotowalk", "scottkelbyphotowalk", "scottkelbysworldwidephotowalk", "scottkelbyworldwidephotowalk", and "scottkelbyssecondannualworldwidephotowalk". So none of those would be allowed (only the original "photowalk"), but "scottkelbyphotowalk2009" would be allowed, since "photowalk" doesn't match the beginning or end of the string.
Or, you could simply do what Stack Overflow does, and limit the number of tags you can enter (they have a limit of 5 per question). I think that would keep people from spamming it so much since they'll have to enter relevant tags, and not just a bunch of variations.
Using your example above, photowalk matches the beginning of "photowalk2009" and the end of "worldwidephotowalk", "scottkelbyphotowalk", "scottkelbysworldwidephotowalk", "scottkelbyworldwidephotowalk", and "scottkelbyssecondannualworldwidephotowalk". So none of those would be allowed (only the original "photowalk"), but "scottkelbyphotowalk2009" would be allowed, since "photowalk" doesn't match the beginning or end of the string.
Or, you could simply do what Stack Overflow does, and limit the number of tags you can enter (they have a limit of 5 per question). I think that would keep people from spamming it so much since they'll have to enter relevant tags, and not just a bunch of variations.