How can I  disable K56 on my Rockwell based modem?

The simple answer is:

This will force the modem into a "pure" V.34 state.  More info below: 

The +MS command is unique to Rockwell V.34 modems and has some unusual qualities. It's important when using this command to replace the entire string. For example, the default V.34 string: +MS=11,1,300,28800 could be tuned to work with a V.22 connection with a string like this: +MS=1,0,300,1200.

Here is the complete definition for V.34>>>

+MS=[<mod>], [<automode>], [<min_rate>], [<max_rate>]

<mod>
n=0  V.21, 300bps
n=1  V.22, 1200bps
n=2  V.22bis, 1200/2400
n=3  V.23 bisynch 75/1200
n=9  V.32, 4800/9600
n=10 V.32bis, 4800/7200/9600/12000/ 14400
n=11  V.34,  2400/4800/7200/9600/ 12000/14400/16800/ 19200/ 24000/26400/ 28800 / 31200/ 33600
n=64  Bell 103, 300bps
n=69  Bell 212, 1200bps
n=74  V.FC,  14400/16800/ 19200/24000/26400/ 28800

<automode>
n=0  Automode Disabled
n=1  Automode enable using V.8 or V.32bis Annex A
note: Automode does not work with Bell 212

<minimum rate>  = 300,1200,2400 etc. default for V.34 is 300bps

<maximum rate>  = 28800,26400 etc. default for V.34 is 28800 or 33600

AT+MS?  Reports MS options

In K56 modems, there are several new fields in the AT+MS command and the field  order has been changed.  The command in K56 chipsets is as follows:

+MS=[<mod>[, [<automode>][, [<min_rate>][, [<max_rate>][, [<x_law>][,
[<reserved>][, [Maxup_rate>]]]]]]

Values are: (0,1,2,3,9,10,11,56,64,69), (0,1), (300-33600), (300-56000), (0,1), (0,1), (300-33600)

<mod>
For K56flex, this field is 56, otherwise it's the same as V.34. Any value other than 56 will disable K56 connections.

<automode>
Same as V.34, but the default is 1

<min_rate>
In K56flex mode, this value is used as in V.34 mode.  If it is set to some value and the line cannot support that value, then the modem will not disconnect like in V.34.

<max_rate>
The max value for this field is 56000. This is useful for restricting the downstream rate. Valid rates are the same as for V.34 with the following additions: 32000/ 34000/ 36000/ 38000/ 40000/ 42000/ 44000/ 46000/ 48000/ 50000/ 52000/ 54000/ 56000

<x_law>
For A-law use 1, for m-law use 0, or let the modem choose automatically. Default is 0.

<reserved>
This field is reserved.  Default is 0. Do not change.

<maxup_rate>
The max value for this field is 33600. This was added so that the max_rate value would not affect asynch K56 connections.