There is a considerable amount of confusion in the information you received. I will go through your message point by point to clarify what you have written.
1) ...after a company has written off a bad debt on their taxes, they are no longer able to collect the debt themselves...
If the original creditor writes-off the debt and issues a 1099-C, that means the original creditor has forgiven the debt and is claiming the loss on its taxes. The 1099-C is a disclosure to the IRS that the borrower has imputed income in the amount listed on the 1099-C. Because the creditor forgave the debt it can no longer be collected upon. This is not a Federal Trade Commission (FTC) rule. Conversely, if there is no 1099-C, then the original creditor has the option to collect the written-off debt itself or it may sell the collection account to a collection agent.
2) ...can no longer charge interest or penalties...
This issue is decided by each state legislature. Some states allow original creditors or collection agents to charge more interest and fees than others. You mentioned you reside in Kansas. See the Bills.com resource Kansas Collection Laws for more information.
3) Can they or can’t they collect on the debt?
Yes, it can collect if there was no 1099-C issued. The creditor may continue to collect on written-off or charged-off debt because forgiven debt is not the same as charged-off or written-off debt. Charged-off or written-off debt may become forgiven, but most is not.
If you are sued by the original creditor or a collection agent, the statute of limitations may be a viable defense. I suggest you speak with an attorney in your state who has experience with debt law to verify your rights.
4) Can they or can’t they charge interest?
To the extent the charging of interest and fees is allowed by your state's laws, yes.
I hope this information helps you Find. Learn & Save.