Error 'exceeded the maximum limit for this feature' when you disable a Partner or Customer Account

This error occurs because the Account being disabled has too many portal-enabled Contact users linked to it. Salesforce enforces a limit on the number of portal-enabled users that can be associated with an Account before it can be disabled.

Partner Accounts

You can disable the Account if it has less than 15 portal enabled Users linked to it (regardless of whether these Users are 'active' or not).

Customer Accounts 

You can disable the Account if it has less than 100 portal enabled Users linked to it (regardless of whether these Users are 'active' or not).

In either case, make sure to first disable the User(s) before disabling the Account. An easy way to do is, by running a report of all Users that are linked to a specific Account, export it and then set the 'IsPortalEnabled' field to False via the Data Loader or another API tool. The Account can be disabled once this has been completed.