Once someone hits a certain WPM...they are moved to a new ranking and they can only compete in typers inthat rank.
You could create typing levels. Each typist would be assigned a level which would depend on his "Average WPM" These levels would be based on the number of words per second (WPS).
There are 4 main steps:
1 WPS - 60 WPM
2 WPS - 120 WPM
3 WPS - 180 WPM
4 WPS - 240 WPM (for gods xD)
And there are 4 intermediate steps:
0,5 WPS - 30 WPM
1,5 WPS - 90 WPM
2,5 WPS - 150 WPM
3,5 WPS - 210 WPM
We can consider that all those who are at the same stage have roughly the same level.
So there would be 9 possible levels that could be called as follows:
Level 0 - 0-29 WPM - Hunt & Peck
Level 1 - 30-59 WPM - Normal speed
Level 2 - 60-89 WPM - Advanced Typing (1 WPS)
Level 3 - 90-119 WPM - Pro Typist
Level 4 - 120-149 WPM - Lightning Fast (2 WPS)
Level 5 - 150-179 WPM - Furious Fingers
Level 6 - 180-209 WPM - Amazing Typist (3 WPS)
Level 7 - 210-239 WPM - Crazy Fingers
Level 8 - 240-255 WPM - Keyboard Master (4 WPS)
These terms are just examples, you can find others. You can also call the level 0 "level 1", whatever.
The idea is to follow the progress of the "Average WPM" so if a typist moves up his average from 50 WPM to 60 WPM, he would go from level 1 to level 2.
One could imagine rankings sorted by level (but keeping the overall), which would give a chance to everyone to be at the top of a ranking. Each typist would therefore appear only in the ranking of his level and in the overall rankings.
The interest would be multiple: it would bring fun and extra motivation. This would better compare typists. This could also add new badges to unlock (1 for each level).
The level would be displayed in the profile (and why not recognizable by a color in rankings and in the last scores). We could be notified by a pop-up when we would gain a level and a difference would be made between primary and intermediate levels.
This could serve to organize battles between typists of the same level.
And finally you could even create special tests that would be unlocked at each level.