Normally to delete objects I just click on them and press the delete key, but there are these really small plants on the ground which are in my way and I cannot click on them, so I cannot delete them. Does anybody know how to?

I have no idea how to remove them, but I think they're connected to the ground plane textures. I just know that when I go into the game, they're not there anymore.

There's a chance it's part of the terrain. With the render window active, press H to open up the terrain editor.

Alternatively, try lassoing the plants. Pants and trees are particularly difficult to select occasionally because you must select the branches or the trunk - if you click the leaves it won't select.