You should be able to go into "Settings" and then "Bluetooth" in there tap the device you are trying to disconnect from and then tap "Forget".
That should disconnect it. If it gives you trouble doing so then try turning off the speaker so that it isn't able to connect, turn restart the phone (turn it off and back on) then try turning on the speaker again. It may just connect. If not, then use the "Forget" option.
Now the strange thing: i CAN get connect to my new clip speaker, but i still can't connect with the larger one that i used to use. Even when i remove the clip on screen the phone doesn't"find" the large speaker. I've cleaned the cache on the phone. About to give up.