You most likely have WSUS configured to automatically approve updates, and to synchronize all products and categories, so it is approving - and therefore downloading and keeping absolutely everything.
In order to remove an update that you no longer need, you must Decline it and then run the cleanup wizard.
There are two sorts of updates you can decline en masse with reasonable safety:
Superseded updates. Go to the All Updates view, select "Any except Declined" and "Any", add the Supersedence column if it isn't already shown, sort by Supersedence, select all the updates that are superseded (these will be in two groups) and decline them.
Updates from classifications you don't need. Create a new update view for the classifications you don't need. From within this view, select "Any except Declined" and "Any", select all updates, and decline them.
Personally, I recommend declining only the Drivers and Driver Sets classifications.
Note that it is not safe to simply decline all updates for Products you do not need, because some updates are for more than one Product. If you decline all updates for Windows 7, for example, you may be inadvertently declining some updates that also apply to Windows 8.1 and/or Windows 10. You can decline updates individually after checking them, but due to the sheer number of updates this would be extremely laborious.
You may wish to review which Products and Classifications are selected for synchronization, to limit the rate at which new content is downloaded, but note that if you choose anything other than "All Products" and "All Classifications" you are then responsible for regularly checking whether there are any new Products and/or Classifications that you will need. You also need to know exactly which products are present on your network, if you leave out a Product you need WSUS will not warn you about it. Also, the Products tree is complex, and in many cases the precise meaning of a particular Product is not immediately obvious.
... I personally configure WSUS for "All Products" and "All Classifications" but I have automatic approvals turned off and only approve updates that are detected as Needed. There are many possible strategies, and only you can decide which is best for you. That said, the simplest strategy, if you can afford to do it, is to leave automatic approvals turned on, continue synchronizing all products and all classifications, and perform a cleanup every few months as described above. This will still take up a sizable chunk of disk space, but not as much as you are using now.