Stacked stone fireplace
Modifying a dry stone wall is a tricky operation. Normally you need two kinds of chisels, a pointing tool and a pitching tool. You can buy them from a masonry supplier like Micon. I would get PKM-19 and SM40-R. Get a mallet too (1031-A).

Grave a deep furrow where you want to break off the stone, set the pitching tool in the furrow, then give it whack. If everything goes well, the stone will split clean off. There is a certain amount of feel that is required. Making sure the furrow is nice and deep will help ensure success.



Why you don't want to use an angle grinder. (In response to comment) The immediate problem with a grinder is that it will make a very unnaturally smooth face that will look nothing like the faces of the other stones, so the ones you cut off will stick out like a sore thumb and look even worse than they do now. The other reason is that cutting rock with an angle grinder, especially indoors, is a bad idea at multiple levels. First of all, if the rock is somewhat hard, like slate, it will take forever. You could have a diamond blade and be standing there cutting for 15 minutes and make it through 1" of stone maybe. Meanwhile you will be surrounded by plumes of dust and toxic fumes as shards of slate go flying in all directions at literally bullet-like speeds.