So when you see the 'blocked sign' (also known as the 'prohibitory sign', 'no entry sign' or 'circle with a line through it') what your Mac is telling you is that it cannot locate a drive with a bootable partition. What this means is that whatever drive the Mac was trying to boot to is either missing a critical boot file or it is damaged (and it either couldn't find another drive to try or it did but that one failed as well). Since you specifically selected the USB drive in an attempt to get it to boot, we know that it is in fact the drive it was trying to boot to. Long story short, your Mac is telling you that the "Bootable USB Drive" you created is not actually bootable.
- Complete an SMC Reset & a PRAM Reset and then try the bootable USB
- If issue still occurs, try to re-create the bootable drive again using the resources you have as it may have experienced an issue/error that wasn't reported to you the first time; then attempt to use the 'new' bootable USB drive again (alternatively, you could try something other than TransMac in case it was the problem)
- If it doesn't work, verify that the Mac still doesn't have a Recovery
Partition option from the Boot disk selection screen (boot holding 'option' key). Unfortunately 99% of Macs from 2009 or earlier don't have the Internet Recovery option no matter which OS they've been upgraded to.
If you have the original install discs that came with your Mac (or a retail version of Snow Leopard), this is where I'd resort to them because my next recommendation can be a headache (or you may simply not have the resources handy). I know it means starting from either Leopard or Snow Leopard and then upgrading, but it would be the easiest and cleanest way to move forward.
If you don't have the original install disc or a retail copy of Snow Leopard, proceed.
Use this section if your Macbook has a firewire port (Early or Mid 2009 model)
If you have access to another Mac you could attempt to put your empty Mac into target disk mode and reformat it using Disk Utility from the functioning Mac (booted normally, not to a Recovery mode). Then you could try to run the El Capitan installer right from there (after placing a copy of it into the /Applications folder), and select your Mac's internal drive as the install destination when prompted. You will need an App Store downloaded copy of the OS installer you're trying to install though, so if you don't have a functioning one you're going to need a solid internet connection to download one.
Apple article for Target Disk Mode: https://support.apple.com/kb/PH22135?viewlocale=en_US&locale=en_US (You will need just a firewire cable if the second Mac has a firewire port; you will also need a Thunderbolt to Firewire adapter if that Mac is too new and does not have a firewire port and instead has a Thunderbolt port)
Skip to this section if your Macbook does not have a firewire port (Late 2009 model)
If your Mac - for whatever reason - will not boot into Target Disk Mode, you could purchase (or borrow) and external drive enclosure, remove the hard drive from your Mac and install it into the drive enclosure, and then plug it in to a functioning Mac via usb. From there you would follow the same steps that I provided for Target Disk Mode - without booting into Target Disk Mode and instead just mounting the drive like a regular external HD - to format and reinstall.