I don't believe there's any cache to clear. The Roku stores nothing in memory but the channel software. If you have a large number of channels that exceeds the internal memory, all it saves is a pointer and will load the channel if called for. It will then remove the least used channel that's in storage. The only way to totally clear the memory is to perform a factory reset, which removes everything but the OS itself.
Dan
Roku Stick (3600), Ultra (4640), Ultra (4670), Ultra (4800), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.