When I turn on my roku it goes directly to Prime Video-I removed this channel and it re-added the channel and does the same thing again.
Try enabling a PIN number from your online Dashboard Account. https://my.roku.com/account and navigate to PIN Preferences. (This should then preclude the Channel from automatically installing without having to input code). Can't explain why self launching on Roku startup unless remote issue (shortcut button), wallpaper/screensaver with Prime programming embedded, or possible software bug as potential reasons.
Also have to verify no one else in house is adding Prime back after you uninstall (all linked Roku devices on same account will mirror installed Channels across the devices - ie installing or uninstalling Channel on one linked device will install or uninstall this Channel on all linked devices).