Please follow the below steps:
Go to the Analysis tab
Choose Estimating
Then select Job Resources on a project and tick Allow Changes,
You can then choose Labour for example, enter 0.00 in the first rate cell
Press F3 to copy down to the next cell. This will make all the rates 0.
Please note that you will have to do it from Job Resources and go through each one, you can't clear it from there as it comes from the Buildup window.