It is easy to get rid of the pop-up but not to have the suspect/infected file sent to the chest. I don't know how you have done this before for the Web Shield as it doesn't have those option in the customize section.
The standard shield does.
See the avast help file, Resident Protection: Standard Shield Provider Settings - "Advanced" Page.
Click on Standard Shield and then on Customize.
Go to Advanced tab and select Silent Mode and the General answer No.
These settings don't exist in the Web Shield.