I don't know if you have ever tried it, but there is an option to try in the free program, after which they would be looking for you to sign up as it isn't a free option.
I don't have Secureline VPN installed, so I'm not familiar with its removal.
You can check if the option is in avast free under the avast UI > Tools.
You can see if you can uninstall it - Control Panel > Programs & Features > Avast Antivirus > Change - from this window select Change if not already there - uncheck the SecurelineVPN if there. If not look for a stand alone SecurelineVPN in Programs & Features and uninstall it.