While I do agree that another horse shouldn't be created only for color, I think we should give the OP the benefit of the doubt. What if this is a nice mare and they just want to know how to avoid another chestnut before they start looking for stallions? Everyone always seems to jump to conclusions on here...
OP, the good news is that it is very easy to avoid getting another chestnut from your mare. You should have plenty of good stallions to choose from. Black is dominant, so if you can find a stallion who is homozygous for black you will have %100 black based foals. That would be the easiest way to avoid a chestnut. A black based horse can be black, bay, brown, etc. so you'd still have lots of possible colors.
Dilution genes can change a chestnut into another color, like palomino (with the cream gene) or red dun (with the dun gene). Classic roan would turn a chestnut into a red roan and grey would give you a grey horse, or course ;) Actually, grey would be another easy way to go, because even if the foal was born chestnut, it would turn white eventually.
Just remember that to have a guarantee that your foal would get a copy of a particular color gene the stallion must be homozygous for it (that is, have two copies of the same gene). Many stallions will be a guarantee of a nice color but also have awful conformation and temperaments, so be sure not to let the promise of color interfere with your choice.
I am here to learn! :)