We can delete the history of the devices but we cannot remove the recent devices from Shareit. But in Bluetooth and we can unpair the paired devices and that won't be visible until we again pair the devices.
As of now, there is no way possible to do that. But you can go to the link SHAREit|Google Play and can add a comment in review section and for this functionality in next version.
Meanwhile goto App info and clear data for SHAREit to remove recent devices. But it will remove every data related to SHAREit ie it will also remove your user and you have to again create a user to use Shareit.