Terry and Vic, it is possible to stop Microsoft from automatically downloading Windows 10 and to clear existing files off your computer.
I held off until January this year from upgrading from Windows 8.1 by hiding the update KB3035583 which is responsible for triggering the download in the background. If you hide it rather than delete it Microsoft will not offer it again which will thwart the download.
The above link itemises other update numbers relating to which operating system you are currently running.
Just for good measure I set the Internet connection as metered which stops Microsoft downloading any updates automatically. Beware though if you do this you will need to ensure that you manually update your system and virus control regularly to stay safe.
I have found Windows 10 faster and easier to use than Windows 8.1....it is up and running in less than 25 seconds, including entering my security code.
When you do install the system spend time going through the settings to turn off Microsofts default 'snooping' settings as they seem to want to know everything about you and your online actions.
After it has settled down it is worth doing a system clean up to remove old operating system files and duplicated updates which are retained on your hard drive. I did this recently and removed 24 gigabytes of unneeded files.
Also it's worth making a recovery memory stick after you have installed Windows10 in case you have any problems starting which a few people have experienced.
As Windows 7 support is due to end in the not too distant future it is not if, but when you accept the update and remember the 'free' offer ends in July.
I'm sure any problems that you may find can be overcome via Google or asking on this and other forums when you decide to update.