you should have volume control at your guitar as well. if the problem is 'guitar sounds ok, but too loud input' that's the way to go.
on the other hand if the signal is moderate, but you have occasional spikes that are much louder, this is another situation. you may use a comp/limiter to cut these spikes. sometimes a hi-pass also will work. or something else, depending on the character and/or nature of these spikes.