how to block ?1251 subject email's

[Obantec Support]

Hi

I run SA2.6.3 with sendmail .

Seem to get quite a few Russian?? emails thru with 

Subject: =?Windows-1251?B?z/Du9OXx8eju7eDr/O3g/yDw5err4OzgIOTr/yDC4PEu?=

Subject is always ?Windows-1251?
followed by the random string/encoded text.

How can i stop these?

Mark