Hi,
the standard lookup for customerid accepts both accounts and contacts.
You have several ways:
1) going unsupported with some javascript code that will modify the DOM in order to select only accounts (NOT SUGGESTED)
2) create a small javascript that will clean the field and advise the user if he chose a contact
3) create a brand new lookup pointing to account put in the form and hide the standard customerid field, after with some javascript, business rule or synchronous workflow copy the value from the new lookup to the customerid one.
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