Just depends on how exactly they were installed:
If they're just stud-mounted in E.I.F.S., then a stiff putty knife works well.
If they're stud-mounted and there's silicone on the backs of the letters, which I assume there probably is, a combination of a thin, narrow, sharp putty knife and a stiff one (for prying) should do the trick. A wire plumbers saw works well sometimes as well.
If they're pad-mounted with silicone, basically the same method as above, minus the wire saw...it usually won't get behind the pads.
Depending on how much silicone was used, there's likely going to be some fascia damage.