Your current renown is divided by the 200 (ie, 0.5%), then the result is subtracted from your current renown. This happens once per week. To eliminate it, simply change the 200 to a very high number, such as 99999.
If you wanted to do it a different way, the 168.000000 is the number of hours between renown decreases (168 hours = once per week by default). Again, you could simply change this to a very high number, such as 9999.000000, and the renown decrease would only happen about once per year.
It occurred to me after my last post that maybe you weren't referring to the weekly renown loss, but rather the renown "subtraction" that happens every time you gain renown (ie, the idea that as your renown gets higher and higher, it also becomes more and more difficult to gain new renown).
If you want to change that part of it, go to your SCRIPTS.TXT file and find the following:
By default, your existing renown is divided by the 200, then the result is subtracted from the original renown gain. For example, if you won a battle with a renown value of 15, and your character already had 2000 renown, you would only actually get +5 renown at the end of the battle (ie, 2000/200 = 10; 15-10=5).
If you wanted to modify the amount of renown that gets subtracted from each renown gain, simply change the 200 to a different value. For example, if you change it to 400, you would still have some renown taken away, but only half as much as usual. Likewise, if you wanted to completely eliminate the renown subtraction, you would change the 200 to a very high value, such as 999999.