Simplifying land cover classification in QGIS?

I have recently completed a land use classification using over 100 ROIs for 5 different land cover classes and it has developed what looks like a highly specific spectral signature for each class. This has meant that the classification is almost like a series of disconnected coloured squares (see pic). It would be great if, for example, similar pixels surrounding or next to a different pixel could convert it to the same class so as to show more contiguous classes. Any ideas?

Green = forest Purple = sub canopy cultivation blue = degraded forest cyan = monoculture yellow = village

enter image description here