iMessage defaults my messages from my iCloud email

I have an ancient iMac that is my main computer. I can't seem to force it to stop using my icould email with iMessage. I have gone to iMessage preferences and removed the "icloud" address. I've put in, following, "Start conversation . . ." with my cell number, but it still won't stop using the icloud address. I've deleted that address on my contact card. I am out of ideas. Would love to fix this.