Setting item permissions question

Hello!

I am trying to create a approval workflow that prevents approvers from modifying items before and after making approval decisions (the approvals are made through the Flexi-Task action)

To do this, I have

1. given all visitors a customised permission level that allows them to add items to a list and view items, but not edit items.  
2. and making use of the item permissions boxes that are available in the configuration window of Flexi-Task, with the following settings

• Set user permissions to: Contribute
• When task is complete, set user permission to: Read

However, with this configuration, the approvers get an "access denied" message when they click on the approval URL. According to the permissions list, the approvers are given "limited access", rather than "contribute".

Would anyone be so kind as to point me to the right directions as to what I need to do to allow approvers to make approvals, but at the same time preventing them from modifying items before and after making approval decisions?

Thank you very much!

Yours faithfully,

Christopher Tse