I quote Joris’ message:
One may use the package ‘invisible-multiply’ to suppress the tiny space (for multiplications) altogether.
In ‘TeXmacs/packages/customize/spacing’, you may find several
other packages to control the spacing in math mode.
See here for a list of packages about controling the spacing.
To add a package, click Document -> Style -> Add package -> Add other packages..., input invisible-multiply (or other package names).