It all depends on what stage of growth your plant is in. If you are still in the vegetative stage (18/6), you can safely top your plant. What that does is when you snip the top of the newly emerging growth on your plant, is that gives energy to the lower branches to create a more bushy like plant. It also splits the top of the plant, into 2 main cola buds, instead of the typical 1 cola. How you top the plant is, is you need clean, very sharp snips, and you cut off about 80% of the new growth coming from the top of the plant.

That will help somewhat with the height, and if your plant is already too tall for your space in the Veg. stage, you are surely screwed for flowering as it easily doubles or triples in height.

If you are flowering though, it is not a good idea to remove any leafs at all, and never cut the top bud off. That might send your plant into shock and you will result in a hermi plant, loaded with seeds.

The only thing you can do is either: Get more space, or you could try to the LST method (Low Stress Training). That is where you bend the main branches over by tying them them down, VERY GENTLY bending the stem.

Other than that, good luck!